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Design and operation of the double-cylinder rotary ash cooler for large CFB boilers
Applied Thermal Engineering 171 (2020) 115056
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Design and operation of the double-cylinder rotary ash cooler for large CFB boilers
T
Huiren Xiao, Man Zhang, Yang Zhang , Yuge Yao, Qing Liu, Hairui Yang, Junfu Lyu ⁎
Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Energy and Power Engineering, Tsinghua University, Haidian District 100084, Beijing, China
HIGHLIGHTS
difference of heat transfer area is the major influence factor of the output. • The between the solids and its covered wall dominates the overall heat transfer. • Contact and operation performance of double-cylinder ash cooler were introduced. • Design • Relation between operating performance and rotating speed was provided. ARTICLE INFO
ABSTRACT
Keywords: Circulating fluidized bed Bottom ash cooler Double-cylinder Thermal performance Heat transfer coefficient
The bottom ash cooler is an important auxiliary equipment of a circulating fluidized bed (CFB) boiler. As the CFB boiler technology develops, the rotary ash coolers have partially substituted the fluidized bed bottom ash coolers in the industrial practice due to its excellent reliability. To meet the increased output requirement, a doublecylinder rotary ash cooler, rather than the conventional single-cylinder one, was proposed in the present study. This paper provided the design of the double-cylinder rotary ash cooler and presented its application in a 300 MWe CFB boiler. The design parameters, structural features, heat transfer performance, as well as the field trial results were discussed in the present study. In addition, a semi-empirical model was proposed to provide theoretical support for the design principle of the double-cylinder rotary ash cooler. The field trial results showed that the actual output of the installed double-cylinder rotary ash cooler was up to 32 t/h with the ash temperature at the outlet being the designed value, much greater than the value (~23 t/h) of the single-cylinder one at approximately the same size. Particularly, this novel bottom ash cooler had run continuously for more than three years without any maintenances due to its reasonable design.
1. Introduction Circulating fluidized bed (CFB) boiler is a combustion device, burning solid fuels and producing steam for heat supply and power generation [1–4]. In a CFB boiler, the solid fuel particles are fluidized and burned in the riser, forming ash particles [5–7]. Fine ash particles, carried by the flue gas, fly out of the boiler through the cyclone separator exit. This part of the ash is called the fly ash [8–10]. Coarse ash particles fall to the lower part of the riser and leave the bed at a relatively high temperature. This part of the ash is the so-called bottom slag [11]. The bottom slag passes through a slag discharge pipe and enters an ash cooler for an exhaustive cooling so that the heat of the bottom slag is recovered and the physical heat loss of the overall process is reduced. The ash cooler is mainly characterized by its bottom slag
⁎
delivery capacity, heat transfer capacity and particle abrasion characteristics [12]. Ash cooler is an important component to ensure the overall performance of a CFB boiler [13]. In the past decades, great progress has been accomplished in the field of CFB boiler technology [14–19]. Now China has the largest market share, capacity and unit number of CFB boilers in the world [20–21]. Meanwhile, various types of ash cooling devices have been developed and installed in China [22]. Currently, two types of ash coolers are frequently adopted: fluidized bed ash cooler and rotary ash cooler. The fluidized bed ash cooler is essentially a small bubbling fluidized bed with the economizer tube group and water-cooled heat transfer surface installed [23–24]. The fluidized bed ash cooler is favored by its large heat transfer coefficient (HTC), strong cooling capacity and large output, and thus it was adopted by most of the early large-scale fluidized bed boilers in China. The performance of the
Corresponding author. E-mail address: [email protected] (Y. Zhang).
https://doi.org/10.1016/j.applthermaleng.2020.115056 Received 7 August 2019; Received in revised form 4 October 2019; Accepted 7 February 2020 Available online 08 February 2020 1359-4311/ © 2020 Elsevier Ltd. All rights reserved.
Applied Thermal Engineering 171 (2020) 115056
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Nomenclature A Cp Cgw d h L Q T u z
σo ε
area specific heat capacity, J/ (kg· K) model parameter diameter, m heat transfer coefficient (HTC), W/ (m2·°C) length, m heat flow rate, W temperature, °C velocity, m/s coordinate axis, m
Super-/Sub-scripts b bl c con f g l m p r s w s-g s-w g-w w-l
Greek letters ρ β λ η θ ω
Stefan-Boltzmann constant emissivity
density, kg/m3 model parameter thermal conductivity, W/(m·K) ratio the radius angle that the slag bed faces, rad rotating speed, r/s
fluidized bed ash cooler highly depends on the fluidization quality. This requires that the particle size distribution of the bottom slag should be within a reasonable range. However, the CFB boilers in China are mainly fed by low-grade coal, whose ash content is high and the slag size covers a very broad range [25–26]. Severe abrasion and poor fluidization problems have been found in the fluidized bed ash cooler in the industrial practice, significantly limiting its output and reducing its reliability. The rotary ash cooler is an alternative and is frequently adopted in the industry. The rotary ash cooler is a moving bed and is categorized as a mechanical power ash cooler. In a rotary ash cooler, the bottom slag is deposited inside a cylinder that equipped with inner fins. The rolling cylinder drives the bottom slag moving forward. The requirement of the bottom slag particle size in the rotary ash cooler is not as strict as that in the fluidized bed ash cooler, so that the rotary ash cooler provides better adaptability and reliability [27]. Generally, the rotary ash cooler is currently more suitable for the CFB boilers in China. The capacity of the early single-cylinder rotary ash cooler strongly depends on the physical scale of the cooler. In the practices, due to the space limit, the output capacity of the single-cylinder rotary ash cooler is already unable to meet the increasing demand of the bottom slag processing.
particle bed thermal boundary layer convective contact fluid gas phase liquid, cooling water mixing particle radiation slag cooling wall slag to gas slag to wall gas to wall wall to cooling water
Consequently, the idea of double-cylinder rotary ash cooler has emerged. Based on the successful experience of the single-cylinder ash cooler, the double-cylinder ash coolers for 300 MWe and 600 MWe CFB boilers have been developed. The operation practices have proved excellent performance and reliability of the double-cylinder ash cooler. A good understanding of the heat transfer process is needed for the characterization and optimization of the double-cylinder ash cooler. As the double-cylinder ash cooler is newly proposed, to the best of the author’s knowledge, neither the analysis on its heat transfer process nor the operation data have been reported in the literature. In this paper, to fill the gap in the literature, the key heat transfer mechanism that dominating the overall heat transfer process was identified. A semi-empirical model was established to analyze the heat transfer process inside the double-cylinder ash cooler. The design and operation performance of the double-cylinder ash cooler of a 300 MWe CFB boiler was introduced and the heat transfer of the double-cylinder rotary ash cooler was discussed. The aim of the present study is to promote the understanding of the heat transfer process inside the double-cylinder ash cooler and logically demonstrate the advantages of the double-cylinder ash cooler in large-scale CFB boiler applications.
Fig. 1. Overall structure of the double-cylinder rotary ash cooler. 2
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2. Structural design of the rotary ash cooler
One should note that similar 1D assumptions are also adopted in the single-cylinder ash cooler analysis. Examples can be found in the literature (e.g. [29–31]).
The structural design of the double-cylinder rotary ash cooler is schematically demonstrated in Fig. 1. The core of the ash cooler is the heat exchanger. The heat exchanger consists of an outer cylinder and a built-in inner cylinder connecting to the branch water pipe and mother water pipe respectively, forming two independent water circuits. The cooling water goes into the sleeve cylinder interlayer and exchanges heat with the slag. The geometry of the cross-section of the heat exchanger is shown in Fig. 2. The cylinder of the ash cooler is installed with fins and tubes. The fins are categorized into two groups. The first group is slag-guide fins whose role is cooling and conveying the slag. The second group is slag-mix fins which mix the hot and cold slag to enhance the heat exchange. Generally, the heat exchange between the slag and the ash cooler occurs on the inner wall of the outer cylinder and the inner/outer wall of the inner cylinder. The slag is discharged from the furnace into the outer cylinder and inner cylinder. Then it is elevated by the slag-mix fins to a higher place inside the cylinder and eventually falls slowly to the bottom. The slag is also pushed by the spiral guide (slag-guide) fins, moving along the direction of the cylinder axis during this period as well. In this double-cylinder design, the heat exchange surface is enlarged in a very limited space. The unique design of the spade-shaped slag-mix fins and the more reasonable structure layout (as shown in Fig. 2) almost double the effective contacting area between the slag and the heat exchange surface (details to be shown in Section 3.4). The output of an ash cooler depends on the heat transfer capability of the equipment. Indeed, there are two factors determine the heat transfer capability, namely, the heat transfer area and the heat transfer coefficient. In Section 3, a semi-quantitatively assessment was conducted to present differences in the heat transfer area and heat transfer coefficient between the single cylinder and double cylinder ash cooler.
3.2. Heat transfer coefficient (HTC) analysis The overall heat transfer from the hot gas-solid flow to the watercooled surface consists of three parts: (1) from the slag particle bed to the slag-covered wall (slag-covered wall HTC); (2) thermal radiation from the slag particle bed to the slag-uncovered wall (slag-uncovered wall HTC); (3) from the hot gas to the slag-uncovered wall (gas-wall HTC). The heat transfer coefficients (HTC) of the three parts are analysis as follows. Slag-covered wall HTC Fig. 4 schematically shows the mechanism of the heat transfer between the slag and its covered wall. Physically, there are three layers of thermal resistances and the detailed explanations of them are discussed as follows. (1) The contact thermal resistance, 1/hg,s. Schlünder thought the contact heat transfer consists of the particle conduction, the gas conduction and the thermal radiation [28]. Based on previous studies [28–31], the contact thermal resistance between the slag and the gas film was calculated by Eq. (1):
dp 1 = hg, s g
(1)
where dp is the particle diameter, β is a model parameter which is determined by the experiments. In this paper β was 0.14 as per a previous study [31].
3. Heat transfer analysis
(2) The thermal resistance of the thermal boundary layer, 1/hbl,s. From Fig. 4, the thermal resistance is essentially a 1D unsteady thermal conduction problem, and can be solved by obtaining the temperature distribution. Generally, the thickness of the thermal boundary layer is much thicker than that of the gas film but much thinner than the depth of ash.
3.1. Heat transfer process analysis Understanding the heat transfer process in a rotary ash cooler is crucial for its design and operation. The basic structure, as well as the physical model of the heat transfer process, are schematically demonstrated in Fig. 3. Essentially, the double cylinder rotary ash cooler can be abstracted into two coaxial water-cooled cylinders with fins installed. The symmetric axis is defined as the z-axis in the present study (Fig. 3a). At a certain axial location (z at a certain value), the crosssection vertical to the z-axis is illustrated in Fig. 3b. Experimental measurements demonstrated that the axial temperature change (from 950 °C to 150 °C) along the cylinder is much greater than the radial temperature change (~60 °C) of the cylinder. This gives a hint that the double cylinder can be simplified to a quasi-one-dimensional structure. Accordingly, the following assumptions are made:
The thermal resistance in the thermal boundary layer was calculated as Eq. (2) [32].
1 = 0.5 hbl, s 2
s s Cp, s
(2)
where λs is the heat conduction coefficient of slag, W/(m·K); ρs is the
(1) Although the hot slag temperature (Ts) on a cross-section (i.e., at a certain z location) is un-uniform, the un-uniformity of Ts at a certain z location is ignored. In other words, Ts satisfies the one-dimensional distribution along the z-axis, namely, Ts = Ts (z). (2) Similarly, the temperatures of the cylinder walls (Tw1 & Tw2), the air (Tg) inside the rotary ash cooler, the cooling water (Tl1 & Tl2) also satisfy the one-dimensional distribution along z-axis, namely, Tw1 = Tw1 (z), Tw2 = Tw2 (z), Tg = Tg (z), Tl1 = Tl1 (z), and Tl2 = Tl2 (z). (3) The heat loss to the environment is ignored. (4) The process is in a steady state. (5) Ts inside the inner cylinder equals to Ts in the channel between the inner and outer cylinders; similar assumption to Tg.
Fig. 2. A sectional view of the double-cylinder heat exchanger. 3
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Fig. 3. Schematics of a double cylinder rotary ash cooler (θ, the radius angle that the slag bed faces).
considerable manner in the real industrial cases, the best approach to reduce 1/hbl,s is to increase the rotating speed ω. 3.2.1. Slag-uncovered wall HTC Assuming the cylinder wall is of a simple cylinder shape (ignoring the fins) and the slag bed surface is a gray body surface, the radiation heat transfer coefficient (HTC) between the slag inside the inner cylinder and the slag-uncovered inner wall of the inner cylinder can be calculated using Eq. (4) [31]:
hrs
w
=
1 1 w
bulk density of slag, kg/m3; Cp,s is the specific heat capacity of slag, kJ/ (kg·K); ω is the rotating speed of cylinder, rad/s; θ is the radius angle that the slag bed faces, rad.
Thus, the HTC between the slag and the wall can be calculated by Eq. (3).
1 +
1 h bl,s
s
1
)
o
Ts4 Ts
T¯w4 T¯w
(4)
3.2.2. Gas-wall HTC The thermal radiation of gas is weak and thereby is ignored. The heat convection between the gas and the water-cooled wall was considered in the present study. As the ash cooler is relatively large in the diameter (~2.25 m), the uncovered wall of the cylinder can be regarded as an infinite-large plate from the heat transfer point of view. In the present study, the convection heat transfer coefficient, hcg-w is calculated using Eq. (5) [34–38].
(3) The thermal resistance of slag mixing, 1/hm,s. Normally, the cylinder rolling results in a rather fast mixing of the slag particles and the thermal resistance caused by slag mixing can be ignored [33].
1 h s,g
(
1
where σo is the Boltzmann constant, 5.67 × 10−8 W/ (m2·K4); εw the blackness of cylinder wall and εs the slag blackness; As-g the bed radiation area and Ag-w the wall radiation area, m2. In the channel between the two cylinders, there is also the thermal radiation from the gas-solid bed surface to the uncovered inner wall of the outer cylinder and the outer wall of the inner cylinder. The thermal radiation HTC here is difficult to precisely calculate due to the complexity of the geometry. Here, Eq. (4) is still employed for a quick estimation of the thermal radiation HTC from the slag to the uncovered wall surface in the channel between the two cylinders. Although this assumption is far from accurate, it still catches the dominating mechanism and is capable of getting the right order of magnitude.
Fig. 4. The heat transfer between the slag and slag-covered wall.
s w hcon =
+
Ag w As g
hcg
(3)
w
= Cg - w u f0.7
(5)
where the Cg-w is a model parameter being 3.38 based on the experimental results in the literature [38]. uf is the gas velocity, m/s. As the temperature of the inner and outer cylinder wall is approximately 100 °C and the temperature difference is small, the radiation heat transfer between the inner and outer cylinder walls is ignored.
One should also note that among the three layers the thermal resistance, 1/hbl,s is the largest one. Naturally, reducing 1/hbl,s is the key to enhance the heat transfer from the slag to the slag-covered wall. According to Eq. (2), 1/hbl,s is influenced by rotating speed (ω) and the radius angle (θ). Since the θ value is difficult to be flexibly tuned in a
4
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3.3. Qualitative estimation of the heat transfer
Comparing the calculation results of Eq. (9) to the one of Eq. (8), the heat transfer area of the double-cylinder structure approximates 1.5 times as the one of the single-cylinder ash cooler at the same volume. It is expected that the double-cylinder ash cooler gives much greater output than the single-cylinder one. In the study, a field trial of the proposed double-cylinder ash cooler design was conducted and the results are discussed in Section 4.
A quick qualitative estimation of the heat transfer is presented in this section in order to explore the mechanism dominating the overall heat transfer. In this estimation, the inner and outer cylinders are simplified as two co-axial cylinders with smooth surfaces with no fins. Thus, the heat transfer between the gas–solid flow and the wall of the cylinders can be calculated as Eqs. (6) and (7): s w = Slag to slag - covered wall: Qcon
i
s w s w hcon As - w,i T = hcon T
Slag to slag - uncovered wall: Qrs - w = Gas to slag - uncovered wall: Qcg - w =
s w Qtotal = Qcon + Qrs
w
+ Qcg
i
i
A s - w,i , i=slag - covered surfaces
h s - w Ag - w,i T , j=slag - uncovered surfaces j r
hcg - w Ag - w,i T = hcg - w T
k
4. Field trial results and discussion
(7)
w
4.1. Specification of the double-cylinder ash coolers
where Q denotes the heat flow rate, W. Table 1 lists the operation parameters of the roller, slag, and air. Table 2 lists the physical properties of slag and air. Using Eqs. (6) and (7), the heat flow rate is calculated and the dominating mechanism is demonstrated in Fig. 5. The η value in Fig. 5, being the ratio of the heat flow rate of each mechanism to the total heat flow rate, denotes the relative contribution of each heat transfer mechanism to the overall heat transfer. Similar to the finding in the single-cylinder ash cooler [31], the contact heat transfer (Qcons,w) from the slag bed to the slag-covered wall overs w whelmingly dominates the overall heat transfer, as con > 90%, as s-w shown in Fig. 5. Qcon increases as the rotating speed increases. The radiation heat flow rate Qrs w shows little change and the rs w decreases with the increase of the rotating speed as the overall heat flow rate increase. The gas to wall heat flow rate is much lower than the other two as the temperature difference between the gas and wall is small. As the contact heat transfer dominates, increasing the contact area is then a key to promote the overall heat transfer capability. In Section 3.4, the contact area is analyzed for a better understanding of the heat transfer process in the double-cylinder ash cooler.
The field trial was conducted in a 300 MWe subcritical CFB boiler in Shanxi Guofeng power plant, China. The designed slag discharge temperature is 950 °C. The boilers are designed to be fueled with low calorific value coal (12248 kJ/kg) with a high ash content (50.6% as received basis). Under the boiler designed condition, the slag discharge rate is 79.49 t/h at the full load. Each boiler was originally equipped with four single-cylinder rotary ash coolers (max. output 23 t/h each). Recently this power plant changes the fuel to a lower caloric value coal (11236 kJ/kg) with higher ash content (57.7% as received basis), and the slag discharge rate of a boiler increases to 116.1 t/h at the full load. The capacity of original four single-cylinder ash coolers cannot meet the new requirement, and thereby four double-cylinder ash coolers (the same outer dimensions as those single-cylinder ones) are proposed and installed. The specifications of the installed double-cylinder rotary ash cooler are given in Table 3. 4.2. Performances of double-cylinder rotary ash cooler The slag output of ash cooler is categorized into the mechanical delivery output and thermal cooling output. The formal one represents the slag delivery capacity and the latter one represents the amount of the slag that can be cooled to the required temperature by the ash cooler. The small value of the two outputs is determined as the final characteristic output, reflecting the actual operating capacity of the ash cooler. In the field trial, the temperature of the slag after cooling was maintained 70–150 °C. The output of the ash cooler was adjusted by tuning the rotating speed of the roller. The operational data of three double-cylinder ash coolers (denoted as 1#, 2# and 3#) was collected. The performance of the double-cylinder ash cooler is demonstrated in Figs. 8 and 9. From Figs. 8 and 9, it can be seen that the output increases and the water temperature decreases with the increase of rotating speed in ash cooler 1# and 2# as expected, but ash cooler 3# reverse. According to the heat transfer analysis in section 3, the output of ash cooler should show an upward tendency and the water
3.4. Heat transfer area analysis In the practice, θ (the radius angle that the bed surface faces) is found almost unchanged as the rotating speed varied. Thus, the contact heat transfer area is proportional to the total heat transfer area. The sectional views of the single-cylinder and double-cylinder ash cooler are depicted in Figs. 6 and 7, respectively. As annotated in Fig. 6, heat transfer area of the single-cylinder ash cooler consists of five parts: (1) the area of the cylinder surface, A1 = πdL; (2) and (3) are the area of fins, A2 + A3 = 8dL; (4) the area of the membrane wall, A4 = 3dL; (5) the equivalent area of fins on the membranes wall, A5 = 6dL. (cylinder diameter is d and cylinder length is L). Thus, the total heat transfer area of the single-cylinder ash cooler:
A=
Ai = ( + 17) dL
20.14dL
(8)
As annotated in Fig. 7, the heat transfer area of double-cylinder ash cooler consists of four parts: (1) the area of the outer cylinder surface, A1 = πd1L = πd1L; (2) the area of inner cylinder surface, A2 = 2πd2L; (3) the area of slag-mix fins, A3 = 1.5πd1L + 3πd2L; and (4) the area of slag-guide fins, A4 = 2πd1L + 2πd2L. (d1: diameter of outer cylinder, d2: diameter of inner cylinder, d2 = 0.8d1). Total heat transfer area of the double-cylinder ash cooler:
A=
Ai = 7 d2 L + 4.5 d1 L = 10.1 d1 L
31.7d1 L
(6)
Ag - w,i , k=slag - uncovered surfaces
Table 1 Properties of the roller, slag, and air.
(9)
5
Project/symbol
value
Project/symbol
value
Slag particle diameter dp
1.07 mm
80 °C
Air average temperature Slag temperature Air suction of ash cooler
100 °C 550 °C 4500 m3/h
Wall average temperature T¯w θ radiation temperature Ts gas velocity uf
2.529 rad 620 °C 0.315 m/s
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Table 2 Physical properties of the slag, and air. Air
50 °C 3
Density(kg/m ) Heat conduction coefficient (W/m·K) Heat capacity (J/kg·K)
500 °C
1.093 2.83 1005
Fig. 5. Effect of rotating speed on the heat transfer ( s w = Qrs w /Qtotal, cg w = Qcg w /Qtotal). r
Slag 3
0.456 5.74 1093
s w con
Bulk Density(kg/m ) Heat conduction coefficient (W/m·K) Heat capacity (J/kg·K)
50 °C
1000 °C
800 0.98 810
900 1.08 980
s-w = Qcon /Qtotal,
Fig. 7. A sectional view of double-cylinder rotary ash cooler. Table 3 Specifications of the double-cylinder rotary ash cooler. Project
Unit
Value
Number Single output Input slag diameter Input slag temperature Output slag temperature Cylinder diameter Distance between the inlet and outlet of slag Effective heat transfer area Input water temperature Output water temperature Total water consumption of ash cooler for single boiler Air suction of ash cooler
– t/h mm °C °C mm m m2 °C °C t/h
4/boiler 3–35 ≤10–40 950 ≤150 2246 10 290 44(winter)/70(summer) 94(winter)/110(summer) 450(winter)/ 570(summer) 4500
m3/h
increased (maximum 32–35 t/h). However, a greater rotating speed may raise greater abrasion problem based on previous experiences. Thus, the rotating speed cannot be unlimitedly increased. In the practice, the rotating speed is controlled within the range of 4–20 rpm as per the slag output requirements. The field trial results confirmed the feasibility and the good performance of the double-cylinder rotary ash cooler. This concept can be applied to a number of CFB boiler systems. More importantly, the double-cylinder rotary ash coolers have been running for more than three years without a major repair, confirming the reasonable design and excellent reliability and of the double-cylinder rotary ash cooler.
Fig. 6. A sectional view of single-cylinder rotary ash cooler.
temperature should show a downward tendency with the increase of rotating speed just like the ash cooler 1# and 2#. The abnormal phenomenon in ash cooler 3# may be raised by the local blockage in the ash cooler and external disturbance according to the analysis of the field trial data. At the rotating speed being 8 rpm, the output of a double-cylinder rotary ash cooler reached 32 t/h, almost 1.4 times as the maximum output (23 t/h) of a single-cylinder rotary ash cooler at the same outer dimensions. These results agree well with the heat transfer estimation in Section 3. By further increasing the rotating speed, the output of the double-cylinder rotary ash cooler further
5. Conclusion In this paper, the design and operation performance of double-cylinder rotary ash cooler were presented. The heat transfer analysis of the rotary ash cooler was given as well. The analysis showed that the 6
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Ye, Study on fluidization characteristics of fluidized bed ash cooler, Tsinghua university, 2006 (in Chinese). [24] J. Chen, G. Tao, X. Lu, et al., Effect of pressure distribution on flow and amount of bed material in selective fluidized bed ash coolers, J. Power Eng. 05 (2007) 677–681 (in Chinese). [25] J. Yang, J. Zhu, Visualization of solids phase separation in a rectangular CFB riser using a novel image calibration method, Powder Technol. 273 (2015) 76–82. [26] J. Yang, J. Zhu, An alternative method to quantify solids phase separation in a narrow rectangular CFB riser, Procedia Eng. 102 (2015) 1064–1072. [27] J. Li, J. Lv, B. Li, Energy and efficiency analysis of ash cooling equipment of 300 MWe circulating fluidized bed unit, J. Combustion Sci. Technol. 17 (4) (2011) 355–362. [28] E.U. Schlünder, Particle heat transfer, Proceedings of the Seventh International Heat Transfer Conference, 1982, pp. 195–211. [29] X. Si, H. Yang, Y. Wu, et al., Heat transfer in the rotary ash cooler with residual char combustion considered, Fuel Process. Technol. 94 (1) (2012) 61–66. [30] F. Herz, I. Mitov, E. Specht, et al., Experimental study of the contact heat transfer coefficient between the covered wall and solid bed in rotary drums, Chem. Eng. Sci.
Fig. 8. The outlet water temperature of the double-cylinder ash cooler as a function of the rotating speed.
Fig. 9. The output of the double-cylinder ash cooler as a function of the rotating speed.
contact heat transfer from the slag-bed to its covered water-cooled wall dominated the overall heat transfer process. The double-cylinder ash cooler almost had a 1.5- times effective heat transfer area as the singlecylinder ash cooler at the same volume. The field trial results demonstrated that the output of the tested double-cylinder ash cooler reached up to 32 t/h, much greater than the output (~23 t/h) of the singlecylinder ash cooler at the same outer size. The reported data in the present study confirmed the reasonable design and excellent reliability of the proposed double-cylinder rotary ash cooler. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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H. Xiao, et al. 82 (2012) 312–318. [31] W. Wang, X. Si, H. Yang, et al., Heat-transfer model of the rotary ash cooler used in circulating fluidized-bed boilers, Energy Fuels 24 (4) (2010) 2570–2575. [32] S. Li, L. Ma, W. Wan, et al., A mathematical model of heat transfer in a rotary kiln thermo-reactor, Chem. Eng. Technol. 28 (12) (2010) 1480–1489. [33] J. Lehmberg, M. Hehl, K. Schgerl, Transverse mixing and heat transfer in horizontal rotary drum reactors, Powder Technol. 18 (2) (1977) 149–163. [34] S.H. Tscheng, A.P. Watkinson, Convective heat transfer in a rotary kiln, Can. J.
Chem. Eng. 57 (4) (2010) 433–443. [35] A.P. Baskakov, B.V. Berg, O.K. Vitt, et al., Heat transfer to objects immersed in fluidized beds, Powder Technol. 8 (5) (1973) 273–282. [36] D. Shi, R. Nicolai, L. Reh, Wall to bed heat transfer in circulating fluidized beds, Chem. Eng. Process. 37 (4) (1998) 287–293. [37] S. Kakac, A. Pramuanjaroenkij, Y. Yener, Convective heat transfer, 1983. [38] M. Magiliotou, Y. Chen, L. Fan, Bed-immersed object heat transfer in a three-phase fluidized bed, AIChE J. 34 (6) (1988) 1043–1047.
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Design and operation of the double-cylinder rotary ash cooler for large CFB boilers
T
Huiren Xiao, Man Zhang, Yang Zhang , Yuge Yao, Qing Liu, Hairui Yang, Junfu Lyu ⁎
Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Energy and Power Engineering, Tsinghua University, Haidian District 100084, Beijing, China
HIGHLIGHTS
difference of heat transfer area is the major influence factor of the output. • The between the solids and its covered wall dominates the overall heat transfer. • Contact and operation performance of double-cylinder ash cooler were introduced. • Design • Relation between operating performance and rotating speed was provided. ARTICLE INFO
ABSTRACT
Keywords: Circulating fluidized bed Bottom ash cooler Double-cylinder Thermal performance Heat transfer coefficient
The bottom ash cooler is an important auxiliary equipment of a circulating fluidized bed (CFB) boiler. As the CFB boiler technology develops, the rotary ash coolers have partially substituted the fluidized bed bottom ash coolers in the industrial practice due to its excellent reliability. To meet the increased output requirement, a doublecylinder rotary ash cooler, rather than the conventional single-cylinder one, was proposed in the present study. This paper provided the design of the double-cylinder rotary ash cooler and presented its application in a 300 MWe CFB boiler. The design parameters, structural features, heat transfer performance, as well as the field trial results were discussed in the present study. In addition, a semi-empirical model was proposed to provide theoretical support for the design principle of the double-cylinder rotary ash cooler. The field trial results showed that the actual output of the installed double-cylinder rotary ash cooler was up to 32 t/h with the ash temperature at the outlet being the designed value, much greater than the value (~23 t/h) of the single-cylinder one at approximately the same size. Particularly, this novel bottom ash cooler had run continuously for more than three years without any maintenances due to its reasonable design.
1. Introduction Circulating fluidized bed (CFB) boiler is a combustion device, burning solid fuels and producing steam for heat supply and power generation [1–4]. In a CFB boiler, the solid fuel particles are fluidized and burned in the riser, forming ash particles [5–7]. Fine ash particles, carried by the flue gas, fly out of the boiler through the cyclone separator exit. This part of the ash is called the fly ash [8–10]. Coarse ash particles fall to the lower part of the riser and leave the bed at a relatively high temperature. This part of the ash is the so-called bottom slag [11]. The bottom slag passes through a slag discharge pipe and enters an ash cooler for an exhaustive cooling so that the heat of the bottom slag is recovered and the physical heat loss of the overall process is reduced. The ash cooler is mainly characterized by its bottom slag
⁎
delivery capacity, heat transfer capacity and particle abrasion characteristics [12]. Ash cooler is an important component to ensure the overall performance of a CFB boiler [13]. In the past decades, great progress has been accomplished in the field of CFB boiler technology [14–19]. Now China has the largest market share, capacity and unit number of CFB boilers in the world [20–21]. Meanwhile, various types of ash cooling devices have been developed and installed in China [22]. Currently, two types of ash coolers are frequently adopted: fluidized bed ash cooler and rotary ash cooler. The fluidized bed ash cooler is essentially a small bubbling fluidized bed with the economizer tube group and water-cooled heat transfer surface installed [23–24]. The fluidized bed ash cooler is favored by its large heat transfer coefficient (HTC), strong cooling capacity and large output, and thus it was adopted by most of the early large-scale fluidized bed boilers in China. The performance of the
Corresponding author. E-mail address: [email protected] (Y. Zhang).
https://doi.org/10.1016/j.applthermaleng.2020.115056 Received 7 August 2019; Received in revised form 4 October 2019; Accepted 7 February 2020 Available online 08 February 2020 1359-4311/ © 2020 Elsevier Ltd. All rights reserved.
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Nomenclature A Cp Cgw d h L Q T u z
σo ε
area specific heat capacity, J/ (kg· K) model parameter diameter, m heat transfer coefficient (HTC), W/ (m2·°C) length, m heat flow rate, W temperature, °C velocity, m/s coordinate axis, m
Super-/Sub-scripts b bl c con f g l m p r s w s-g s-w g-w w-l
Greek letters ρ β λ η θ ω
Stefan-Boltzmann constant emissivity
density, kg/m3 model parameter thermal conductivity, W/(m·K) ratio the radius angle that the slag bed faces, rad rotating speed, r/s
fluidized bed ash cooler highly depends on the fluidization quality. This requires that the particle size distribution of the bottom slag should be within a reasonable range. However, the CFB boilers in China are mainly fed by low-grade coal, whose ash content is high and the slag size covers a very broad range [25–26]. Severe abrasion and poor fluidization problems have been found in the fluidized bed ash cooler in the industrial practice, significantly limiting its output and reducing its reliability. The rotary ash cooler is an alternative and is frequently adopted in the industry. The rotary ash cooler is a moving bed and is categorized as a mechanical power ash cooler. In a rotary ash cooler, the bottom slag is deposited inside a cylinder that equipped with inner fins. The rolling cylinder drives the bottom slag moving forward. The requirement of the bottom slag particle size in the rotary ash cooler is not as strict as that in the fluidized bed ash cooler, so that the rotary ash cooler provides better adaptability and reliability [27]. Generally, the rotary ash cooler is currently more suitable for the CFB boilers in China. The capacity of the early single-cylinder rotary ash cooler strongly depends on the physical scale of the cooler. In the practices, due to the space limit, the output capacity of the single-cylinder rotary ash cooler is already unable to meet the increasing demand of the bottom slag processing.
particle bed thermal boundary layer convective contact fluid gas phase liquid, cooling water mixing particle radiation slag cooling wall slag to gas slag to wall gas to wall wall to cooling water
Consequently, the idea of double-cylinder rotary ash cooler has emerged. Based on the successful experience of the single-cylinder ash cooler, the double-cylinder ash coolers for 300 MWe and 600 MWe CFB boilers have been developed. The operation practices have proved excellent performance and reliability of the double-cylinder ash cooler. A good understanding of the heat transfer process is needed for the characterization and optimization of the double-cylinder ash cooler. As the double-cylinder ash cooler is newly proposed, to the best of the author’s knowledge, neither the analysis on its heat transfer process nor the operation data have been reported in the literature. In this paper, to fill the gap in the literature, the key heat transfer mechanism that dominating the overall heat transfer process was identified. A semi-empirical model was established to analyze the heat transfer process inside the double-cylinder ash cooler. The design and operation performance of the double-cylinder ash cooler of a 300 MWe CFB boiler was introduced and the heat transfer of the double-cylinder rotary ash cooler was discussed. The aim of the present study is to promote the understanding of the heat transfer process inside the double-cylinder ash cooler and logically demonstrate the advantages of the double-cylinder ash cooler in large-scale CFB boiler applications.
Fig. 1. Overall structure of the double-cylinder rotary ash cooler. 2
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2. Structural design of the rotary ash cooler
One should note that similar 1D assumptions are also adopted in the single-cylinder ash cooler analysis. Examples can be found in the literature (e.g. [29–31]).
The structural design of the double-cylinder rotary ash cooler is schematically demonstrated in Fig. 1. The core of the ash cooler is the heat exchanger. The heat exchanger consists of an outer cylinder and a built-in inner cylinder connecting to the branch water pipe and mother water pipe respectively, forming two independent water circuits. The cooling water goes into the sleeve cylinder interlayer and exchanges heat with the slag. The geometry of the cross-section of the heat exchanger is shown in Fig. 2. The cylinder of the ash cooler is installed with fins and tubes. The fins are categorized into two groups. The first group is slag-guide fins whose role is cooling and conveying the slag. The second group is slag-mix fins which mix the hot and cold slag to enhance the heat exchange. Generally, the heat exchange between the slag and the ash cooler occurs on the inner wall of the outer cylinder and the inner/outer wall of the inner cylinder. The slag is discharged from the furnace into the outer cylinder and inner cylinder. Then it is elevated by the slag-mix fins to a higher place inside the cylinder and eventually falls slowly to the bottom. The slag is also pushed by the spiral guide (slag-guide) fins, moving along the direction of the cylinder axis during this period as well. In this double-cylinder design, the heat exchange surface is enlarged in a very limited space. The unique design of the spade-shaped slag-mix fins and the more reasonable structure layout (as shown in Fig. 2) almost double the effective contacting area between the slag and the heat exchange surface (details to be shown in Section 3.4). The output of an ash cooler depends on the heat transfer capability of the equipment. Indeed, there are two factors determine the heat transfer capability, namely, the heat transfer area and the heat transfer coefficient. In Section 3, a semi-quantitatively assessment was conducted to present differences in the heat transfer area and heat transfer coefficient between the single cylinder and double cylinder ash cooler.
3.2. Heat transfer coefficient (HTC) analysis The overall heat transfer from the hot gas-solid flow to the watercooled surface consists of three parts: (1) from the slag particle bed to the slag-covered wall (slag-covered wall HTC); (2) thermal radiation from the slag particle bed to the slag-uncovered wall (slag-uncovered wall HTC); (3) from the hot gas to the slag-uncovered wall (gas-wall HTC). The heat transfer coefficients (HTC) of the three parts are analysis as follows. Slag-covered wall HTC Fig. 4 schematically shows the mechanism of the heat transfer between the slag and its covered wall. Physically, there are three layers of thermal resistances and the detailed explanations of them are discussed as follows. (1) The contact thermal resistance, 1/hg,s. Schlünder thought the contact heat transfer consists of the particle conduction, the gas conduction and the thermal radiation [28]. Based on previous studies [28–31], the contact thermal resistance between the slag and the gas film was calculated by Eq. (1):
dp 1 = hg, s g
(1)
where dp is the particle diameter, β is a model parameter which is determined by the experiments. In this paper β was 0.14 as per a previous study [31].
3. Heat transfer analysis
(2) The thermal resistance of the thermal boundary layer, 1/hbl,s. From Fig. 4, the thermal resistance is essentially a 1D unsteady thermal conduction problem, and can be solved by obtaining the temperature distribution. Generally, the thickness of the thermal boundary layer is much thicker than that of the gas film but much thinner than the depth of ash.
3.1. Heat transfer process analysis Understanding the heat transfer process in a rotary ash cooler is crucial for its design and operation. The basic structure, as well as the physical model of the heat transfer process, are schematically demonstrated in Fig. 3. Essentially, the double cylinder rotary ash cooler can be abstracted into two coaxial water-cooled cylinders with fins installed. The symmetric axis is defined as the z-axis in the present study (Fig. 3a). At a certain axial location (z at a certain value), the crosssection vertical to the z-axis is illustrated in Fig. 3b. Experimental measurements demonstrated that the axial temperature change (from 950 °C to 150 °C) along the cylinder is much greater than the radial temperature change (~60 °C) of the cylinder. This gives a hint that the double cylinder can be simplified to a quasi-one-dimensional structure. Accordingly, the following assumptions are made:
The thermal resistance in the thermal boundary layer was calculated as Eq. (2) [32].
1 = 0.5 hbl, s 2
s s Cp, s
(2)
where λs is the heat conduction coefficient of slag, W/(m·K); ρs is the
(1) Although the hot slag temperature (Ts) on a cross-section (i.e., at a certain z location) is un-uniform, the un-uniformity of Ts at a certain z location is ignored. In other words, Ts satisfies the one-dimensional distribution along the z-axis, namely, Ts = Ts (z). (2) Similarly, the temperatures of the cylinder walls (Tw1 & Tw2), the air (Tg) inside the rotary ash cooler, the cooling water (Tl1 & Tl2) also satisfy the one-dimensional distribution along z-axis, namely, Tw1 = Tw1 (z), Tw2 = Tw2 (z), Tg = Tg (z), Tl1 = Tl1 (z), and Tl2 = Tl2 (z). (3) The heat loss to the environment is ignored. (4) The process is in a steady state. (5) Ts inside the inner cylinder equals to Ts in the channel between the inner and outer cylinders; similar assumption to Tg.
Fig. 2. A sectional view of the double-cylinder heat exchanger. 3
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Fig. 3. Schematics of a double cylinder rotary ash cooler (θ, the radius angle that the slag bed faces).
considerable manner in the real industrial cases, the best approach to reduce 1/hbl,s is to increase the rotating speed ω. 3.2.1. Slag-uncovered wall HTC Assuming the cylinder wall is of a simple cylinder shape (ignoring the fins) and the slag bed surface is a gray body surface, the radiation heat transfer coefficient (HTC) between the slag inside the inner cylinder and the slag-uncovered inner wall of the inner cylinder can be calculated using Eq. (4) [31]:
hrs
w
=
1 1 w
bulk density of slag, kg/m3; Cp,s is the specific heat capacity of slag, kJ/ (kg·K); ω is the rotating speed of cylinder, rad/s; θ is the radius angle that the slag bed faces, rad.
Thus, the HTC between the slag and the wall can be calculated by Eq. (3).
1 +
1 h bl,s
s
1
)
o
Ts4 Ts
T¯w4 T¯w
(4)
3.2.2. Gas-wall HTC The thermal radiation of gas is weak and thereby is ignored. The heat convection between the gas and the water-cooled wall was considered in the present study. As the ash cooler is relatively large in the diameter (~2.25 m), the uncovered wall of the cylinder can be regarded as an infinite-large plate from the heat transfer point of view. In the present study, the convection heat transfer coefficient, hcg-w is calculated using Eq. (5) [34–38].
(3) The thermal resistance of slag mixing, 1/hm,s. Normally, the cylinder rolling results in a rather fast mixing of the slag particles and the thermal resistance caused by slag mixing can be ignored [33].
1 h s,g
(
1
where σo is the Boltzmann constant, 5.67 × 10−8 W/ (m2·K4); εw the blackness of cylinder wall and εs the slag blackness; As-g the bed radiation area and Ag-w the wall radiation area, m2. In the channel between the two cylinders, there is also the thermal radiation from the gas-solid bed surface to the uncovered inner wall of the outer cylinder and the outer wall of the inner cylinder. The thermal radiation HTC here is difficult to precisely calculate due to the complexity of the geometry. Here, Eq. (4) is still employed for a quick estimation of the thermal radiation HTC from the slag to the uncovered wall surface in the channel between the two cylinders. Although this assumption is far from accurate, it still catches the dominating mechanism and is capable of getting the right order of magnitude.
Fig. 4. The heat transfer between the slag and slag-covered wall.
s w hcon =
+
Ag w As g
hcg
(3)
w
= Cg - w u f0.7
(5)
where the Cg-w is a model parameter being 3.38 based on the experimental results in the literature [38]. uf is the gas velocity, m/s. As the temperature of the inner and outer cylinder wall is approximately 100 °C and the temperature difference is small, the radiation heat transfer between the inner and outer cylinder walls is ignored.
One should also note that among the three layers the thermal resistance, 1/hbl,s is the largest one. Naturally, reducing 1/hbl,s is the key to enhance the heat transfer from the slag to the slag-covered wall. According to Eq. (2), 1/hbl,s is influenced by rotating speed (ω) and the radius angle (θ). Since the θ value is difficult to be flexibly tuned in a
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3.3. Qualitative estimation of the heat transfer
Comparing the calculation results of Eq. (9) to the one of Eq. (8), the heat transfer area of the double-cylinder structure approximates 1.5 times as the one of the single-cylinder ash cooler at the same volume. It is expected that the double-cylinder ash cooler gives much greater output than the single-cylinder one. In the study, a field trial of the proposed double-cylinder ash cooler design was conducted and the results are discussed in Section 4.
A quick qualitative estimation of the heat transfer is presented in this section in order to explore the mechanism dominating the overall heat transfer. In this estimation, the inner and outer cylinders are simplified as two co-axial cylinders with smooth surfaces with no fins. Thus, the heat transfer between the gas–solid flow and the wall of the cylinders can be calculated as Eqs. (6) and (7): s w = Slag to slag - covered wall: Qcon
i
s w s w hcon As - w,i T = hcon T
Slag to slag - uncovered wall: Qrs - w = Gas to slag - uncovered wall: Qcg - w =
s w Qtotal = Qcon + Qrs
w
+ Qcg
i
i
A s - w,i , i=slag - covered surfaces
h s - w Ag - w,i T , j=slag - uncovered surfaces j r
hcg - w Ag - w,i T = hcg - w T
k
4. Field trial results and discussion
(7)
w
4.1. Specification of the double-cylinder ash coolers
where Q denotes the heat flow rate, W. Table 1 lists the operation parameters of the roller, slag, and air. Table 2 lists the physical properties of slag and air. Using Eqs. (6) and (7), the heat flow rate is calculated and the dominating mechanism is demonstrated in Fig. 5. The η value in Fig. 5, being the ratio of the heat flow rate of each mechanism to the total heat flow rate, denotes the relative contribution of each heat transfer mechanism to the overall heat transfer. Similar to the finding in the single-cylinder ash cooler [31], the contact heat transfer (Qcons,w) from the slag bed to the slag-covered wall overs w whelmingly dominates the overall heat transfer, as con > 90%, as s-w shown in Fig. 5. Qcon increases as the rotating speed increases. The radiation heat flow rate Qrs w shows little change and the rs w decreases with the increase of the rotating speed as the overall heat flow rate increase. The gas to wall heat flow rate is much lower than the other two as the temperature difference between the gas and wall is small. As the contact heat transfer dominates, increasing the contact area is then a key to promote the overall heat transfer capability. In Section 3.4, the contact area is analyzed for a better understanding of the heat transfer process in the double-cylinder ash cooler.
The field trial was conducted in a 300 MWe subcritical CFB boiler in Shanxi Guofeng power plant, China. The designed slag discharge temperature is 950 °C. The boilers are designed to be fueled with low calorific value coal (12248 kJ/kg) with a high ash content (50.6% as received basis). Under the boiler designed condition, the slag discharge rate is 79.49 t/h at the full load. Each boiler was originally equipped with four single-cylinder rotary ash coolers (max. output 23 t/h each). Recently this power plant changes the fuel to a lower caloric value coal (11236 kJ/kg) with higher ash content (57.7% as received basis), and the slag discharge rate of a boiler increases to 116.1 t/h at the full load. The capacity of original four single-cylinder ash coolers cannot meet the new requirement, and thereby four double-cylinder ash coolers (the same outer dimensions as those single-cylinder ones) are proposed and installed. The specifications of the installed double-cylinder rotary ash cooler are given in Table 3. 4.2. Performances of double-cylinder rotary ash cooler The slag output of ash cooler is categorized into the mechanical delivery output and thermal cooling output. The formal one represents the slag delivery capacity and the latter one represents the amount of the slag that can be cooled to the required temperature by the ash cooler. The small value of the two outputs is determined as the final characteristic output, reflecting the actual operating capacity of the ash cooler. In the field trial, the temperature of the slag after cooling was maintained 70–150 °C. The output of the ash cooler was adjusted by tuning the rotating speed of the roller. The operational data of three double-cylinder ash coolers (denoted as 1#, 2# and 3#) was collected. The performance of the double-cylinder ash cooler is demonstrated in Figs. 8 and 9. From Figs. 8 and 9, it can be seen that the output increases and the water temperature decreases with the increase of rotating speed in ash cooler 1# and 2# as expected, but ash cooler 3# reverse. According to the heat transfer analysis in section 3, the output of ash cooler should show an upward tendency and the water
3.4. Heat transfer area analysis In the practice, θ (the radius angle that the bed surface faces) is found almost unchanged as the rotating speed varied. Thus, the contact heat transfer area is proportional to the total heat transfer area. The sectional views of the single-cylinder and double-cylinder ash cooler are depicted in Figs. 6 and 7, respectively. As annotated in Fig. 6, heat transfer area of the single-cylinder ash cooler consists of five parts: (1) the area of the cylinder surface, A1 = πdL; (2) and (3) are the area of fins, A2 + A3 = 8dL; (4) the area of the membrane wall, A4 = 3dL; (5) the equivalent area of fins on the membranes wall, A5 = 6dL. (cylinder diameter is d and cylinder length is L). Thus, the total heat transfer area of the single-cylinder ash cooler:
A=
Ai = ( + 17) dL
20.14dL
(8)
As annotated in Fig. 7, the heat transfer area of double-cylinder ash cooler consists of four parts: (1) the area of the outer cylinder surface, A1 = πd1L = πd1L; (2) the area of inner cylinder surface, A2 = 2πd2L; (3) the area of slag-mix fins, A3 = 1.5πd1L + 3πd2L; and (4) the area of slag-guide fins, A4 = 2πd1L + 2πd2L. (d1: diameter of outer cylinder, d2: diameter of inner cylinder, d2 = 0.8d1). Total heat transfer area of the double-cylinder ash cooler:
A=
Ai = 7 d2 L + 4.5 d1 L = 10.1 d1 L
31.7d1 L
(6)
Ag - w,i , k=slag - uncovered surfaces
Table 1 Properties of the roller, slag, and air.
(9)
5
Project/symbol
value
Project/symbol
value
Slag particle diameter dp
1.07 mm
80 °C
Air average temperature Slag temperature Air suction of ash cooler
100 °C 550 °C 4500 m3/h
Wall average temperature T¯w θ radiation temperature Ts gas velocity uf
2.529 rad 620 °C 0.315 m/s
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Table 2 Physical properties of the slag, and air. Air
50 °C 3
Density(kg/m ) Heat conduction coefficient (W/m·K) Heat capacity (J/kg·K)
500 °C
1.093 2.83 1005
Fig. 5. Effect of rotating speed on the heat transfer ( s w = Qrs w /Qtotal, cg w = Qcg w /Qtotal). r
Slag 3
0.456 5.74 1093
s w con
Bulk Density(kg/m ) Heat conduction coefficient (W/m·K) Heat capacity (J/kg·K)
50 °C
1000 °C
800 0.98 810
900 1.08 980
s-w = Qcon /Qtotal,
Fig. 7. A sectional view of double-cylinder rotary ash cooler. Table 3 Specifications of the double-cylinder rotary ash cooler. Project
Unit
Value
Number Single output Input slag diameter Input slag temperature Output slag temperature Cylinder diameter Distance between the inlet and outlet of slag Effective heat transfer area Input water temperature Output water temperature Total water consumption of ash cooler for single boiler Air suction of ash cooler
– t/h mm °C °C mm m m2 °C °C t/h
4/boiler 3–35 ≤10–40 950 ≤150 2246 10 290 44(winter)/70(summer) 94(winter)/110(summer) 450(winter)/ 570(summer) 4500
m3/h
increased (maximum 32–35 t/h). However, a greater rotating speed may raise greater abrasion problem based on previous experiences. Thus, the rotating speed cannot be unlimitedly increased. In the practice, the rotating speed is controlled within the range of 4–20 rpm as per the slag output requirements. The field trial results confirmed the feasibility and the good performance of the double-cylinder rotary ash cooler. This concept can be applied to a number of CFB boiler systems. More importantly, the double-cylinder rotary ash coolers have been running for more than three years without a major repair, confirming the reasonable design and excellent reliability and of the double-cylinder rotary ash cooler.
Fig. 6. A sectional view of single-cylinder rotary ash cooler.
temperature should show a downward tendency with the increase of rotating speed just like the ash cooler 1# and 2#. The abnormal phenomenon in ash cooler 3# may be raised by the local blockage in the ash cooler and external disturbance according to the analysis of the field trial data. At the rotating speed being 8 rpm, the output of a double-cylinder rotary ash cooler reached 32 t/h, almost 1.4 times as the maximum output (23 t/h) of a single-cylinder rotary ash cooler at the same outer dimensions. These results agree well with the heat transfer estimation in Section 3. By further increasing the rotating speed, the output of the double-cylinder rotary ash cooler further
5. Conclusion In this paper, the design and operation performance of double-cylinder rotary ash cooler were presented. The heat transfer analysis of the rotary ash cooler was given as well. The analysis showed that the 6
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Fig. 8. The outlet water temperature of the double-cylinder ash cooler as a function of the rotating speed.
Fig. 9. The output of the double-cylinder ash cooler as a function of the rotating speed.
contact heat transfer from the slag-bed to its covered water-cooled wall dominated the overall heat transfer process. The double-cylinder ash cooler almost had a 1.5- times effective heat transfer area as the singlecylinder ash cooler at the same volume. The field trial results demonstrated that the output of the tested double-cylinder ash cooler reached up to 32 t/h, much greater than the output (~23 t/h) of the singlecylinder ash cooler at the same outer size. The reported data in the present study confirmed the reasonable design and excellent reliability of the proposed double-cylinder rotary ash cooler. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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