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A study on the adaptability of cyclone barrel to slag film in a cyclone-fired boiler
Accepted Manuscript A Study on the Adaptability of Cyclone Barrel to Slag Film in a Cyclone-fired Boiler Song Wu, Lei Deng, Chang'an Wang, Chunli Tang, Yufan Bu, Defu Che PII: DOI: Reference:
S1359-4311(17)32467-5 http://dx.doi.org/10.1016/j.applthermaleng.2017.04.043 ATE 10192
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
24 May 2016 30 March 2017 12 April 2017
Please cite this article as: S. Wu, L. Deng, C. Wang, C. Tang, Y. Bu, D. Che, A Study on the Adaptability of Cyclone Barrel to Slag Film in a Cyclone-fired Boiler, Applied Thermal Engineering (2017), doi: http://dx.doi.org/10.1016/ j.applthermaleng.2017.04.043
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A Study on the Adaptability of Cyclone Barrel to Slag Film in a Cyclone-fired Boiler
Song Wu, Lei Deng, Chang’an Wang, Chunli Tang, Yufan Bu, Defu Che
State Key Laboratory of Multiphase Flow in Power Engineering, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China
To whom correspondence should be addressed. Tel.: +86-029-82665185; fax: +86-029-82668703.
E-mail address: [email protected] (D. Che).
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Abstract: The behavior of slag film inside the cyclone barrel is closely related to its design and operation. In this paper, a multilayer wall model considering the heat transfer from flame to working fluid in the water-cooled tubes was proposed and a judgment criterion was established to evaluate the adaptability of cyclone barrel to slag film. Furthermore, parametric investigation was carried out systematically to analyze the influencing factors of the adaptability, including design parameters (barrel diameter, thickness of refractory lining), operating conditions (excess air ratio, temperature of preheated air) and coal properties (ash content, coal slag temperature of critical viscosity). The results indicate that the molten slag can plug the slag tap at low loads while erode and corrode the refractory lining severely at high loads for a cyclone barrel. There is a load range within which the cyclone barrel operates safely and steadily. The thickness of refractory lining and coal slag temperature of critical viscosity are the key factors affecting the adaptability significantly. The adaptability descends as the thickness of refractory lining increases and a lower coal slag temperature of critical viscosity leads to a decrease in the adaptability. The effects of other factors are relatively small. Especially for barrel diameter, its impact can be disregarded. The conclusions in the present study are helpful to the optimization design and safety operation of cyclone barrels. Keywords: cyclone-fired boiler, cyclone barrel, slag film, heat transfer, thermal performance, adaptability
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1. Introduction The cyclone furnace concept was first proposed by the Babcock & Wilcox Company in the 1940s [1]. In general, a cyclone furnace boiler consists of one or several cyclone barrels in which pulverized coal burns strongly at high temperatures (> 1650 °C) and the coal ash is melted. With the swirling motion of the secondary air, a molten slag layer forms and coats the inside surface of the cyclone barrel. The slag is discharged through the slag tap and enters the slag tank. Cyclone furnaces are widely used for utility boilers, industrial boilers or kilns. Especially, they are extremely suitable for firing coals with low ash melting points, anthracites and high-sodium coals [2]. However, due to the high combustion temperature, relatively high NOx emissions were observed in cyclone-fired boilers, which made the cyclone units less popular. With the development of combustion technology, considerable reductions in NOx levels have been demonstrated in some cyclone boilers using air staging or reburning in recent years [1, 3-7]. As a result, the cyclone-fired boilers gain the comparable competitiveness with the pulverized coal boilers. Both the plugging of slag tap and the excessive erosion/corrosion of refractory lining are the serious problems for cyclone furnaces, which can result in the equipment damages and unit outages [8, 9]. They are bound up with the formation, flow and heat transfer of slag film inside the cyclone barrel. The adaptability of cyclone barrel to slag film represents the ability of cyclone barrel to operate well without the occurrence of plugging and excessive erosion/corrosion. Obviously, the adaptability is determined by the behavior of slag film.
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Many research efforts have been devoted to the behavior of slag film [10-25]. Some researchers [10-13] experimentally simulated the capture efficiencies and flow characteristics of slag by the use of the imitations of molten slag such as water spray, paraffin and syrup. But these studies were all performed in cold model experiments and couldn’t reveal the actual slag film behavior with heat transfer. Cheng et al. [14] detected the deposition and flow of slag in a lab-scale entrained-flow gasifier visually with the help of heat resisting endoscope and digital image processing technology. The results suggested that slag flow velocity has a positive correlation with slag surface temperature and reaches about 0.0026 to 0.003 m/s under the experimental conditions. Moreover, many investigators [15-24] employed the numerical simulation methods to evaluate the behavior of slag film. Seggiani [15] proposed a slag building model in a Prenflo coal gasifier to describe the time varying behavior of the slag layer. The results demonstrated that the model could predict the slag behavior reasonably. A steady-state model considering the particle collision and capture was presented in the work of Yong et al. [16]. Their research showed that particle momentum has a significant effect on the build-up of slag layer. Ye et al. [17, 18] developed a new numerical model to overcome the limitations of existing models and examined the impacts of critical viscosity and its temperature on the behavior of slag film. There were also some researchers utilizing the commercial computational fluid dynamics codes to predict the formation, flow and heat transfer of slag film, and satisfactory results were obtained [21-24]. These models and conclusions are useful to gasifiers but can not be extrapolated to cyclone boilers directly because of lacking sufficient
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validation. In addition, Ryzhakov [25] investigated the effect of slag film on transient processes in the cyclone barrel and reported that the self-regulation in the thickness of slag layer plays an important role in the response of thermal performance when the operating conditions change. These efforts are valuable for understanding the behavior of slag film. However, there is little literature available regarding how slag film works when the plugging or excessive erosion/corrosion occurs in a cyclone furnace. Meanwhile, the adaptability of cyclone barrel to slag film, which is of great significance for the design and operation of cyclone barrels, is still unclear and insufficiently understood. Therefore, it’s necessary to seek a way to analyze and assess the adaptability to guide engineering practice. Furthermore, since the adaptability depends on a number of factors, such as design parameters, operating conditions and coal properties, it’s more desirable to study the influences of these factors systematically. The purpose of this study is to propose an appropriate method to predict the adaptability of cyclone barrel to slag film. To do so, a multilayer wall model of heat transfer was developed to calculate the thermal performance of slag film, and a judgment criterion was presented to evaluate whether the plugging and excessive erosion/corrosion occur or not. Then, this method was employed to analyze a cyclone barrel in detail. Moreover, the effects of various factors on the adaptability were also investigated, including barrel diameter, thickness of refractory lining, excess air ratio, temperature of preheated air, ash content and coal slag temperature of critical viscosity.
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2. Method of Evaluation 2.1. Multilayer wall model of heat transfer As shown in Fig. 1, the cyclone barrel is of water-cooled tube construction. Short pin studs are welded on the outside surface of the tubes in a very dense pattern and a refractory material is installed around the pin studs. In this way a refractory lining is generated. During the operation process, the refractory lining needs to undergo the thermal stress of high temperature and the erosion and corrosion caused by the molten slag. Though the refractory lining faces an extreme working environment, the solid slag layer is able to protect it from the erosion and corrosion. This protection mechanism is called “using slag to prevent slag”.
Fig. 1. Cyclone barrel pin stud and refractory section
In the actual process, the slag layer on the refractory lining can be subdivided into a liquid slag layer, a plastic flow layer and a solid slag layer. The interface between the liquid slag layer and the plastic flow layer is determined by the coal slag temperature of critical viscosity ( t0 ). The molten slag could be considered as a Newtonian fluid when its temperature is above t0 , while as a plastic fluid if the temperature is below
t0 . Generally, the movement of the liquid slag layer is slight [14]. The corresponding Reynolds number is < 0.03, meaning that the flow pattern of the liquid slag layer belongs to laminar flow. Thus, conduction along the normal direction basically controls the heat transfer in the liquid slag layer. Moreover, compared with the barrel diameter (inside diameter of cyclone barrel), the average thickness of liquid slag layer
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is so thin that the process of heat transfer can be treated as a heat conduction problem in the plane wall [26]. The slag flow in the plastic region is neglected due to its high viscosity. The plastic flow layer and solid slag layer can be considered together as one layer here, still referred to as solid slag layer. The refractory lining is composed of metal pin studs and refractory which have different thermal conductivities. In the present study, an averaged heat flux was employed to describe the heat conduction in the refractory lining with a composite thermal conductivity adopted. The assumption of plane wall is also suitable for the solid slag layer and refractory lining. Here, a multilayer wall model of heat transfer is proposed, as illustrated in Fig. 2. The whole process of heat transfer from flame to working fluid is taken into account in the model. When the coal particles are burning, heat is transferred from the high temperature flame to the fireside surface of liquid slag layer by radiation and convection. Then, the heat flows through the liquid slag layer, solid slag layer, refractory lining, and metal tube wall in turn by conduction and reaches the inside surface of the metal tube. Finally, the working fluid absorbs the heat by convection. The scale deposit is negligible since the rigorous water treatment and boiling out previous to operation are always performed. Additionally, the following assumptions are applied in the present model: (1) All the calculation and analysis are carried out in steady-state conditions. (2) The heat transfer is normal to the plane wall. (3) The surface temperatures of various layers are uniform. (4) The temperature profiles in both liquid and solid slag layers are linear.
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(5) The thicknesses of both liquid and solid slag layers are considered uniform.
Fig. 2. Multilayer wall model of heat transfer in the cyclone barrel
Actually, the slag film thickness is not uniform along the perimeter of the cyclone barrel because of gravity. As shown in Fig. 3, for the liquid slag film on the left side, the tangential component of gravity is in the direction of flue gas flow, which can accelerate the circumferential flow of liquid slag. Conversely, the tangential component of gravity is in the opposite direction of flue gas flow and the circumferential flow of liquid slag is inhibited on the right side. Under such a difference, the liquid slag of left side moves faster than that of right side in the circumferential direction. As a result, the slag film on the left side is thinner than that on the right side [27]. Nevertheless, the difference in the slag film thickness is not significant due to the scouring action of fast rotating flame flow on the surface of slag film. The scouring action can form a uniformly distributed tangential force driving the rotating flow of slag film. The tangential force includes the viscous force of gas phases and the impact force of particles. The circumferential behavior of slag film is the result of the joint action of the tangential force and gravity. The tangential force is much larger than the gravity and plays a major role in the circumferential flow of slag film. Therefore, the gravity can only affect the circumferential distribution of the slag film thickness slightly. Furthermore, with the dip angle ( ) increasing, the gravity component in the cross section of the cyclone barrel is reduced. Correspondingly, the effect of gravity on the slag film thickness along the circumferential direction
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weakens. Specially, when the cyclone barrel is arranged perpendicularly, i.e., the vertical cyclone furnace, the gravity will lose its influence power on the circumferential behavior of slag film completely. In summary, the slag film thickness along the perimeter of the cyclone barrel can be regarded as uniform distribution approximately without significant differences introduced, and the assumption of uniform slag film thickness is reasonable in the present study.
Fig. 3. Schematic diagram of liquid slag film along the perimeter of the cyclone barrel
2.2. Thermal performance calculation In accordance with the model in Fig. 2, heat transfer in the multilayer wall of cyclone barrel can be expressed by: qx
tzm t0 t0 trl trl two two twi twi tg Rzm,l Rzm,s Rrl Rw Rc,g
(1)
where qx is the averaged heat flux in the multilayer wall, tzm , trl , two , t wi and tg are the temperatures of liquid slag layer fireside surface, refractory lining fireside surface, water-cooled tube outside surface, water-cooled tube inside surface and saturated working fluid respectively, Rzm,l , Rzm,s , Rrl , Rw and Rc,g are the heat resistances of liquid slag layer, solid slag layer, refractory lining, metal tube wall and convection between working fluid and tube inside surface respectively. The flame and the cyclone barrel enclosure can be considered to be two infinite parallel plates. According to the Stefan-Boltzmann law, heat transfer of radiation in the cyclone barrel is formulated as:
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Qr =
0 aK H r Bj
4 (Ty4 Tzm )
(2)
where Qr is the quantity of radiant heat transfer, 0 is the Stefan-Boltzmann constant, aK is the system emissivity of cyclone barrel, H r is the area of effective radiative heating surface, Ty is the effective average temperature of flame in K, Tzm is the temperature of liquid slag layer fireside surface in K, and Bj is the calculation fuel consumption rate for cyclone barrel. Dividing by B j means the heat transfer is computed based on the calculation fuel consumption of unit mass. The system emissivity of cyclone barrel is defined as: aK
1 1 1 x( 1) aw ay
(3)
where aw and ay are the emissivities of furnace wall and flame respectively, and
x is the averaged angular factor of cyclone barrel, which is calculated as [28]: x
x F F i
bi
(4)
bi
where xi and Fbi are the angular factor and furnace wall area of zone i respectively. For the cyclone barrel, the angular factor of barrel wall is 1, and those of burner and secondary air ports are taken as 0. The temperature of flue gas varies along the cyclone barrel length. In this study, a characteristic temperature, i.e., the effective average temperature of flame, is employed to describe the radiant heat transfer in the cyclone barrel. The effective average temperature of flame is valued as:
Ty 0.925 TaTg,e
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(5)
where Ta is the adiabatic flame temperature in K, and Tg,e is the cyclone barrel exit gas temperature in K. Detailed information on the explanation, derivation and validation of Eq. (5) is provided in the Part 1 of the Supplementary Materials. Owing to the strong rotation motion inside the cyclone barrel, the convection action should be considered. Heat transfer of convection in the cyclone barrel is represented by: Qc
U c H r (Ty Tzm ) Bj
(6)
where Qc is the quantity of convective heat transfer, and U c is the convective heat transfer coefficient. For the convective heat transfer process in the cyclone barrel, it is hard to measure or describe it separately. In order to predict the heat transfer by convection accurately, a modeling test was conducted to seek the convective heat transfer correlation [29]. According to the similarity theory, the Reynolds number was selected as the scaling criterion. Keeping the Reynolds number identical, the similar flow field can be created in the modeling test setup. The test conditions were designed in accordance with the range of the actual operating conditions for cyclone barrels. Through the statistical analyses on a great number of experimental data, the practical correlation is derived as:
Nuc 0.0074Rec
(7)
where Nuc is the Nusselt number, and Rec is the Reynolds number. The barrel diameter is used as the feature size in both of the criterion numbers. They are computed in Eq. (8) and Eq. (9): 42
Nuc
Uc D
(8)
Rec
w2 D
(9)
y
y
where D is the barrel diameter, y is the thermal conductivity of flue gas, w2 is the secondary air velocity, and y is the kinematic viscosity of flue gas. To get tzm or Tzm , an implicit equation is given as Eq. (10). The standard method for thermal calculation of former Soviet Union provides a nomogram to solve it [29]. More information on the solution procedure of the nomogram is provided in the Part 2 of the Supplementary Materials.
tzm t0 f (Ty , t0 , v , A, w2 ) Ty
(10)
where v is the viscosity property factor of slag, and A is the parameter reflecting the thickness of liquid slag layer. They are defined in Eq. (11) and Eq. (12): ln
v
0
t t0
A 0.467 3
G0 0s u s2 ps
(11)
(12)
where 0 is the critical viscosity of slag, is the viscosity of slag at the temperature of t , G0 is the ash mass flow rate entering cyclone barrel, s is the slag capture efficiency, u is the cyclone barrel perimeter wetted by slag, s is the density of slag, and ps is the parameter reflecting the dynamic impact of flue gas on slag film. The slag capture efficiency represents the mass fraction of bottom slag in the total ash of fuel. In cyclone furnaces, 70% to 85% of the fuel ash is captured by
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the barrel wall and the remainder is carried by flue gas to enter the convection pass [1, 29]. Therefore, s is in the range of 0.7 - 0.85 and generally selected as 0.8 in the design or analysis. Since the inside surface of the cyclone barrel is completely coated by a molten slag layer under normal operation, the cyclone barrel perimeter wetted by slag is virtually the inside perimeter of the cyclone barrel ( D ). Furthermore, heat balance in the cyclone barrel is written as:
Qr Qc (Qa I g,e )
(13)
where is the heat retention factor, Qa is the available heat entering cyclone barrel, and I g,e is the cyclone barrel exit gas enthalpy. The heat retention factor represents the fraction of the heat transferred to working fluid through the heating surface in the heat released by flue gas ( 1.0 ), that is to say, most part of the heat released by flue gas is transferred to working fluid, and the remainder is lost to the environment through the outer surface of the boiler setting. is calculated as follows:
1
q5 b q5
(14)
where q5 is the radiation and convection heat loss and b is the thermal efficiency of a boiler. By coupling with the heat transfer in the multilayer wall, qx can be given as:
qx
Bj (Qr Qc ) Hr
(15)
In addition to the above equations, Eq. (16) is supplemented to solve Eq. (1):
R
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(16)
where R is the heat resistance, is the layer thickness, and is the thermal conductivity. Eq. (16) works within all the layers, including liquid slag layer, solid slag layer, refractory lining and metal tube wall. Especially, the composite thermal conductivity of refractory lining ( rl ) can be calculated as:
rl
r m
f r (1 f )m
(17)
where r and m are the thermal conductivities of refractory and metal pin studs respectively, and f is the arrangement density of pin studs. In the present study, the refractory material is of SiC and the pin studs are of 12Cr1MoV alloy steel. According to the operating temperature ranges of the refractory and metal pin studs, their thermal conductivities are 5.814 and 30.232 W/(m·K) respectively, and f is a structure parameter designed as 0.2. With all the equations solved simultaneously, the thermal performances of slag film under given conditions can be derived, such as boundary temperatures of each layer, slag layer thicknesses, averaged heat flux and so on, which will be of help to analyze the adaptability of cyclone barrel to slag film. 2.3. Judgment criterion of adaptability Based on the obtained thermal performances, a judgment criterion of adaptability is presented to evaluate the operation state of cyclone barrel. When the operating conditions, load or fuel changes, the t0 interface in Fig. 2 will move accordingly. In particular, the solid slag layer disappears with the t0 interface moving left and arriving at the fireside surface of refractory lining, which makes the cyclone barrel lose the protection mechanism. In other words, when the thickness of solid slag layer 45
( zm,s ) becomes zero, the molten slag would erode and corrode the refractory lining severely. Furthermore, considering the requirement of slag tapping, the cyclone barrel exit gas temperature ( g,e ) should be always maintained higher than t0 to prevent the slag from plugging the tap. As a result, the judgment criterion is given as follows:
g,e t0 & zm,s 0
(18)
2.4. Method validation In order to verify the validity of the method used in this study, a comparison of the cyclone barrel exit gas temperatures obtained through the existing test data, computational fluid dynamics (CFD) and method of the present study was carried out, as shown in Fig. 4. Three cyclone barrels with different capacities are available to provide the test data and the numerical calculation data. The CFD study was performed in a vertical cyclone barrel with a heat release rate of 61.3 MW, which can give the numerical results of the temperature distribution along the cyclone barrel length (see Part 1 of the Supplementary Materials). According to the comparative results, the maximum deviation of the method used in the present study is 29.3 °C for the cyclone barrel exit gas temperature. Such an agreement is quite satisfactory from an engineering perspective. Since the cyclone barrel exit gas temperature is determined by the heat transfer process in the cyclone barrel, the agreement in the cyclone barrel exit gas temperature demonstrates the validity in the prediction of the heat transfer in the cyclone barrel, and the method of the present study is creditable and reasonable.
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Fig. 4. Comparison of the cyclone barrel exit gas temperatures obtained through the existing test data (or CFD) and method of the present study
Above CFD simulation was carried out by using the commercial software Fluent. The conservation equations of mass, momentum, and energy are solved by the SIMPLE algorithm. In order to model the strong swirling flow in the cyclone barrel properly, the standard k-ε model is chosen to consider the turbulence flow of gases. Particles are modelled with the discrete phase (DPM) model, and particle trajectories are calculated through the Lagrangian approach. Radiation heat transfer is described by the discrete ordinate (DO) model. The absorption coefficient is obtained by the weighted-sum-of-gray-gas (WSGG) model. The probability density function (PDF) is employed to simulate the homogeneous combustion. In addition, the single-step model
is
applied
to
describe
the
coal
devolatilization
and
the
kinetics/diffusion-limited model is used for the char combustion. These models are all widely used in the furnace simulation, which ensures the reliability of the numerical results. 3. Results and Discussion 3.1. Load characteristics According to the above model and formulas, the thermal performances of a cyclone barrel under various heat release rates per unit barrel cross-sectional area ( qF ) within a range of 4000 - 40000 kW/m2 are calculated. The coal property analysis of selected fuel is shown in Table 1. The main design parameters and operating conditions of the cyclone barrel are listed in Table 2.
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Table 1 Coal property analysis of selected fuel (wt %)
Table 2 Main design parameters and operating conditions of a cyclone barrel
The load characteristics of the cyclone barrel are illustrated in Fig. 5. The fuel consumption rate for cyclone barrel ( B ) is proportional to qF in Fig. 5(a). Because the operation load depends on B , qF can be utilized to represent the level of operation load. As shown in Fig. 5(b), qx increases with qF rising while the increase range reduces, indicating that the heat absorbed by the fireside surface of liquid slag layer from flame is raised as the operation load rises. The increase of flame temperature leads to the rise in qx . The reduction of the increase range is attributed to the decrease in the heat transfer quantity which is computed according to the calculation fuel consumption of unit mass. Variations in the surface temperatures of different layers are illustrated in Fig. 5(c). With the operation load rising, there is a slight increase in t wi . Because the boiling heat transfer occurs in the water-cooled tubes, Rc,g is quite small and t wi is close to tg . Moreover, two presents a similar tendency with t wi due to the large enough thermal conductivity of metal. As qF rises, trl increases and a decrease in zm,s is observed in Fig. 5(d), which shows a good agreement with the research results of Ryzhakov [25]. Especially, when qF rises closely to 40000 kW/m2, trl is equal to t0 and zm,s becomes zero simultaneously. The cyclone barrel can’t meet the judgment criterion of adaptability under this condition. Thus, the maximum operation load for safety is derived. g,e
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reduces as qF decreases. With qF falling closely to 4000 kW/m2, g,e equals t0 and the slag tap may become plugged. Therefore, the minimum operation load for safety can be obtained. In addition, it can be seen from Fig. 5(c) and Fig. 5(d) that tzm and the thickness of liquid slag layer ( zm,l ) have a slight variation with the changing operation load owing to the self-regulation of zm,s [18].
Fig. 5. Load characteristics of the cyclone barrel: (a) fuel consumption rate; (b) averaged heat flux; (c) surface temperature distributions of different layers; (d) thicknesses of liquid and solid slag layers; ( D = 2.4 m, L = 9.6 m, rl = 20 mm, = 1.1, tph = 400 °C and qF0 = 20000 kw/m2)
3.2. Effects of design parameters Fig. 6 compares the adaptabilities of various cyclone barrels with a range of 1.6 3.2 m in barrel diameter ( D ). The length/diameter ratio ( L /D ) is chosen as 4.0 and the values of other parameters are the same as those listed in Tables 1 and 2. Fig. 6 indicates that the thermal performances of the cyclone barrels exhibit the similar variation tendencies as qF changes. For a given qF , qx increases with the increasing D . An increase in qx results in a rise in trl and a reduction in g,e .
zm,s decreases as D increases in Fig. 6(d). When qF is increased, the larger D is, the sooner zm,s becomes zero. Therefore, the load range under which the cyclone barrel can maintain security and stability is narrowed as D rises. In other words, the adaptability of cyclone barrel to slag film reduces with D increasing. Noticeably, the variations in the thermal performances are quite small for a given qF , e.g., the maximum temperature difference in g,e is < 30 °C during the whole load range. The
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thermal performances at low loads change little with D . Thus, the effect of barrel diameter on the adaptability is considered unremarkable.
Fig. 6. Effect of barrel diameter on the adaptability: (a) averaged heat flux; (b) temperature of refractory lining fireside surface; (c) cyclone barrel exit gas temperature; (d) thickness of solid slag layer; ( L /D = 4.0, rl = 20 mm, = 1.1, tph = 400 °C and qF0 = 20000 kw/m2)
Fig. 7 shows the effect of thickness of refractory lining ( rl ) on the adaptability. The composite thermal conductivity of refractory lining is valued as 6.934 W/(m·K) according to Eq. (17). Within a range of 10 - 30 mm in rl , the other parameters are shown in Tables 1 and 2. In practice, the solid slag layer can be considered as the extension of refractory lining. For a given operation condition, if the overall heat resistance of refractory lining and solid slag layer is kept constant, the cyclone barrel will work invariably regardless of their thicknesses. The variation in rl is offset by the self-regulation of zm,s . As rl increases, zm,s decreases due to the left movement of the t0 interface (see Fig. 2) and vice versa. Thus, rl mainly affects the solid slag layer while the liquid slag layer is hardly affected. Accordingly, the heat transfer between flame and fireside surface of liquid slag layer is also uninfluenced, as illustrated in Fig. 7(a) and Fig. 7(c). trl increases with rl rising when a qF is given in Fig. 7(b). It can be seen from Fig. 7(d) that zm,s decreases with the increasing rl , demonstrating the adaptability of cyclone barrel declines. Moreover,
rl has a significant effect on the adaptability. When rl is increased from 20 to 25 mm, the load range for safety is cut by about half ( q1 ), and q2 is less than q1 .
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As discussed above, the refractory lining can not be designed too thick or thin. The adaptability will not meet the requirement of load regulation if the refractory lining is too thick, while the wear resistance is not enough if too thin.
Fig. 7. Effect of thickness of refractory lining on the adaptability: (a) averaged heat flux; (b) temperature of refractory lining fireside surface; (c) cyclone barrel exit gas temperature; (d) thickness of solid slag layer; ( D = 2.4 m, L = 9.6 m, = 1.1, tph = 400 °C and qF0 = 20000 kw/m2)
3.3. Effects of operating conditions Fig. 8 presents the effect of excess air ratio ( ) on the adaptability. With varying from 1.05 to 1.2, the other parameters are listed in Tables 1 and 2. Fig. 8(a) indicates that qx falls as rises for a given qF . This is because the temperature of preheated air is far lower than that of the combustion product. An increase in the amount of preheated air leads to a decrease in the flame temperature, which reduces the quantity of heat transfer in the cyclone barrel. In accordance with the variation of
qx , trl falls with increasing in Fig. 8(b). A transition point appears at the qF of 10600 kW/m2 in Fig. 8(c). For qF below 10600 kW/m2, g,e increases as rises. Inversely, g,e declines as rises when qF is above 10600 kW/m2. With an increase of , a decrease in the quantity of heat transfer results in an increase in g,e , while the blend of the increased preheated air and high temperature combustion product can cause a reduction simultaneously. The actual g,e is determined by the comprehensive result of these two effects. The heat transfer plays a dominant role at
51
low loads while the blend of different enthalpies possesses a leading position at high loads. As shown in Fig. 8(d), zm,s increases with
rising, meaning the
adaptability of cyclone barrel rises. In addition, the cyclone barrel utilizing air staging technology is also analyzed. With an excess air ratio of 0.8, partial gasification occurs in the cyclone barrel during the combustion of pulverized coal. The curves for of 0.8 are quite close to those for of 1.2 owing to the accordance in qx . The incomplete combustion of fuel reduces the flame temperature, leading to a decrease in
qx compared with other situations.
Fig. 8. Effect of excess air ratio on the adaptability: (a) averaged heat flux; (b) temperature of refractory lining fireside surface; (c) cyclone barrel exit gas temperature; (d) thickness of solid slag layer; ( D = 2.4 m, L = 9.6 m, rl = 20 mm, tph = 400 °C and qF0 = 20000 kw/m2)
The effect of temperature of preheated air ( tph ) on the adaptability is shown in Fig. 9. Within a range of 280 - 440 °C in tph , the other parameters are listed in Tables 1 and 2. As tph rises, the flame temperature increases and the heat transfer in the cyclone furnace is enhanced. As a result, qx rises with the increasing tph for a given qF in Fig. 9(a). Correspondingly, trl increases with tph rising in Fig. 9(b). The impact of tph on g,e is similar to that of . The enhanced heat transfer results in a decrease in g,e , while the increased enthalpy of air leads to an increase simultaneously. Since the blend of different enthalpies possesses a leading position at high loads, g,e rises as tph increases in Fig. 9(c). It can be seen from Fig. 9(d) that
zm,s reduces with the increasing tph and the adaptability of cyclone barrel decreases
52
accordingly.
Fig. 9. Effect of temperature of preheated air on the adaptability: (a) averaged heat flux; (b) temperature of refractory lining fireside surface; (c) cyclone barrel exit gas temperature; (d) thickness of solid slag layer; ( D = 2.4 m, L = 9.6 m, rl = 20 mm, = 1.1 and qF0 = 20000 kw/m2)
3.4. Effects of coal properties Fig. 10 illustrates the effect of ash content ( Aar ) on the adaptability. Four bituminous coals are investigated and the ultimate analyses are listed in Table 3. For the studied coals, the ash contents are 9.68%, 19.78%, 30.00% and 39.69% respectively and the maximum difference between moisture contents is < 2%. The properties of coal slag are shown in Table 1 and the other parameters are listed in Table 2. When a qF is given, the more Aar is, the more the yield of molten slag is. An increase in the yield of molten slag can thicken the liquid slag layer, resulting in a rise of tzm . Therefore, the heat transfer from the high temperature flame to the fireside surface of liquid slag layer weakens and qx decreases, as shown in Fig. 10(a). Accordingly, trl falls as Aar increases in Fig. 10(b). A transition point is observed at the qF of 23000 kW/m2 in Fig. 10(c). When qF is below 23000 kW/m2,
g,e rises with the increasing Aar . However, g,e falls with the increase of Aar for qF above 23000 kW/m2. As Aar increases, a decrease in qx causes a rise in g,e , while a decrease of adiabatic flame temperature can lead to a reduction simultaneously. The relative strength of the two effects is able to account for the
53
actual g,e . As illustrated in Fig. 10(d), with Aar increasing, zm,s rises and the adaptability of cyclone barrel increases.
Fig. 10. Effect of ash content on the adaptability: (a) averaged heat flux; (b) temperature of refractory lining fireside surface; (c) cyclone barrel exit gas temperature; (d) thickness of solid slag layer; ( D = 2.4 m, L = 9.6 m, rl = 20 mm, = 1.1, tph = 400 °C and qF0 = 20000 kw/m2) Table 3 Ultimate analyses of four studied bituminous coals (wt %)
Fig. 11 describes the effect of coal slag temperature of critical viscosity ( t0 ) on the adaptability. Within a range of 1150 - 1450 °C in t0 , the other parameters are listed in Tables 1 and 2. qx reduces with t0 rising for a given qF in Fig. 11(a). The higher
t0 is, the higher tzm is. Therefore, the temperature difference of heat transfer from the high temperature flame to the fireside surface of liquid slag layer decreases as t0 increases, and the quantity of heat transfer reduces accordingly. In accordance with the variation of qx , trl falls with the increasing t0 in Fig. 11(b). A decrease in qx can lead to an increase in g,e . Therefore, the higher t0 is, the higher g,e is. According to the judgment criterion of adaptability for slag tapping, the minimum operation load for safety changes slightly with t0 , as shown in Fig. 11(c). Furthermore, the flame temperature will be increased when the coals with high ash melting points are fired, admitting the cyclone furnaces to deal with more coals effectively. zm,s rises with the increasing t0 in Fig. 11(d) [15, 18], indicating the adaptability of cyclone barrel increases. It is worth noticing that the effect of t0 is
54
quite remarkable. When t0 rises from 1150 to 1250 °C, the load range for safety is extended by about 80% ( q1 ), and q2 is more than q1 . If the adaptability is too small to meet the operation requirement for the coals with lower ash melting points, reducing thickness of refractory lining or increasing the arrangement density of pin studs will be applied for improvement.
Fig. 11. Effect of coal slag temperature of critical viscosity on the adaptability: (a) averaged heat flux; (b) temperature of refractory lining fireside surface; (c) cyclone barrel exit gas temperature; (d) thickness of solid slag layer; ( D = 2.4 m, L = 9.6 m, rl = 20 mm, = 1.1, tph = 400 °C and
qF0 = 20000 kw/m2)
4. Conclusions In this study, a multilayer wall model of heat transfer from flame to working fluid in the cyclone barrel was proposed and a judgment criterion was given to evaluate the adaptability of cyclone barrel to slag film. Then, the load characteristics and various influencing factors of the adaptability were probed in detail. The following conclusions can be drawn: The cyclone barrel exit gas temperature declines as the operation load falls, and the thickness of solid slag layer decreases as the operation load rises. There is a load range for the cyclone barrel to meet the judgment criterion of adaptability. The adaptability is affected by the thickness of refractory lining and coal slag temperature of critical viscosity significantly, while the effects of other factors are relatively small. Especially, the impact of barrel diameter can be neglected. The adaptability decreases with the thickness of refractory lining increasing, and the
55
influence on heat transfer can be offset by the self-regulation in the thickness of solid slag layer. A lower coal slag temperature of critical viscosity also results in a reduction of the adaptability. Since the adaptability of cyclone barrel to slag film depends on many influential factors simultaneously, further research on multiparameter combined optimization is needed. Acknowledgements The authors acknowledge financial support from the National Key Technology Research and Development Program of China (2015BAA04B02) and the National Natural Science Foundation of China (No. 51506163).
Nomenclature A
parameter reflecting the thickness of liquid slag layer [m]
Aar
ash content [%]
As,i
barrel inside surface area [m2]
a
emissivity [-]
B
fuel consumption rate for cyclone barrel [kg/s]
D
barrel diameter [m]
Fb
furnace wall area [m2]
f
arrangement density of pin studs [-]
G0
ash mass flow rate entering cyclone barrel [kg/s]
Hr
area of effective radiative heating surface [m2]
I g,e
cyclone barrel exit gas enthalpy [J/kg]
56
L
barrel length [m]
Nuc
Nusselt number [-]
pg
saturated working fluid pressure [MPa]
ps
parameter reflecting the dynamic impact of flue gas on slag film [-]
Q
quantity of heat [J/kg]
q5
radiation and convection heat loss [%]
qF
heat release rate per unit barrel cross-sectional area [kW/m2]
qx
averaged heat flux in the multilayer wall [W/m2]
R
heat resistance [m2·K/W]
Rec
Reynolds number [-]
T
temperature [K]
t
temperature [°C]
t0
coal slag temperature of critical viscosity [°C]
Uc
convective heat transfer coefficient [W/(m2·K)]
u
cyclone barrel perimeter wetted by slag [m]
V
barrel volume [m3]
w2
secondary air velocity [m/s]
wu
circumferential velocity of flue gas [m/s]
x
angular factor [-]
x
averaged angular factor of cyclone barrel [-]
Greek letters
excess air ratio [-]
57
v
viscosity property factor of slag [1/°C]
dip angle [°]
qi
shift in the load range for safety [kW/m2]
layer thickness [m]
b
thermal efficiency of a boiler [%]
s
slag capture efficiency [-]
g,e
cyclone barrel exit gas temperature [°C]
thermal conductivity [W/(m·K)]
viscosity of slag at the temperature of t [Pa·s]
0
critical viscosity of slag [Pa·s]
y
kinematic viscosity of flue gas [m2/s]
s
density of slag [kg/m3]
0
Stefan-Boltzmann constant [W/(m2·K4)]
heat retention factor [-]
Subscripts ar
as-received base
c
convection
daf
dry ash-free base
g
working fluid
g,e
cyclone barrel exit gas
i
index for selected object
m
metal pin studs
58
ph
preheated air
r
radiation
rl
refractory lining
s
slag
s,i
inside surface
wi
water-cooled tube inside surface
wo
water-cooled tube outside surface
y
flue gas
zm,l
liquid slag layer
zm,s
solid slag layer
References [1] J.B. Kitto, S.C. Stultz, Steam – Its Generation and Use, 41th ed., Babcock & Wilcox Company, Charlotte, NC, 2005. [2] S. Wu, W. Bai, C. Tang, X. Tan, C.a. Wang, D. Che, A novel boiler design for high-sodium coal in power generation, in: ASME 2015 Power Conference, POWER 2015, collocated with the ASME 2015 9th International Conference on Energy Sustainability, the ASME 2015 13th International Conference on Fuel Cell Science, Engineering and Technology, and the ASME 2015 Nuclear Forum, June 28, 2015 - July 2, 2015, American Society of Mechanical Engineers (ASME), San Diego, CA, United states, 2015, pp. Power Division. [3] H. Farzan, R.J. Newell, G.J. Maringo, J.D. Riggs, A.S. Yagiela, Evaluation of reburning NOx control and coal switching in a cyclone boiler, in: Proceedings of the Engineering Foundation Conference on Coal-Blending and Switching of Low-Sulfur Western Coals, September 26, 1993 October 1, 1993, Publ by ASME, Snowbird, UT, USA, 1994, pp. 323. [4] H. Farzan, G.J. Maringo, C.T. Beard, G.E. Weed, J. Pratapas, NO x control using natural gas reburn on an industrial cyclone boiler, in: Proceedings of the 1997 22nd International Technical Conference on Coal Utilization & amp; Fuel Systems, March 16, 1997 - March 19, 1997, Coal & 59
Slurry Technology Assoc, Clearwater, FL, USA, 1997, pp. 801-814. [5] B. Smith, S. Voss, T. Titus, Overfire air blows away cyclone NO x, Power Engineering (Barrington, Illinois), 103(1) (1999). [6] H. Gadalla, N. Peters, F. Iman, M. Zhanhua, SmartBurn approach for reduced NO x emissions below 0.20 lb/MMBtu at cyclone-fired boilers, in: COAL-GEN 2007 Conference, August 1, 2007 - August 3, 2007, Unavailable, Milwaukee, WI, United states, 2007. [7] W. Bai, H. Li, L. Deng, H. Liu, D. Che, Air-staged combustion characteristics of pulverized coal under high temperature and strong reducing atmosphere conditions, Energy & Fuels, 28(3) (2014) 1820-1828. [8] C.H. Marston, Thermal interactions in the slag tap of a cyclone coal combustor, in: Heat Transfer in Fire and Combustion Systems. Presented at the 23rd National Heat Transfer Conference., ASME, Denver, CO, USA, 1985, pp. 251-258. [9] Z. Zhou, Y. Bo, Y. Zhang, Z. Huang, L. Chen, L. Ge, J. Zhou, K. Cen, Interactions of high-chromia refractory materials with infiltrating coal slag in the oxidizing atmosphere of a cyclone furnace, Ceramics International, 40(3) (2014) 3829-3839. [10] K. Ohtake, Y. Nakatake, Cold model study of flow field and slag rejection efficiency in cyclone slagging combustor, in: Proceedings of the 3rd ASME/JSME Thermal Engineering Joint Conference Part 5 (of 5), March 17, 1991 - March 22, 1991, Publ by ASME, Reno, NV, USA, 1991, pp. 239-244. [11] Y. Nakatake, I. Naruse, K. Ohtake, Cold model experiment of cyclone slagging gasifier with high slag rejection efficiency and flow field control, Transactions of the Japan Society of Mechanical Engineers, Part B, 61(585) (1995) 1928-1934. [12] H. Yuan, H. Qu, H. Ren, F. Wang, Z. Yu, An experimental study of slag deposit in the entrained-flow gasifier, Journal of East China University of Science and Technology, 31(3) (2005) 393-397 (in Chinese). [13] J. Zhang, Q. Liang, J. Wang, J. Xu, H. Liu, X. Gong, Flow characteristics of slag in Shell gasifier slag bath, Chemical Engineering (China), 39(4) (2011) 89-93 (in Chinese). [14] X. Cheng, G. Hou, Q. Liang, J. Xu, H. Liu, Experimental study of slag flow on membrane wall in entrained-flow gasifier, Chemical Engineering (China), 40(3) (2012) 58-62 (in Chinese). [15] M. Seggiani, Modelling and simulation of time varying slag flow in a Prenflo entrained-flow 60
gasifier, Fuel, 77(14) (1998) 1611-1621. [16] S.Z. Yong, M. Gazzino, A. Ghoniem, Modeling the slag layer in solid fuel gasification and combustion – Formulation and sensitivity analysis, Fuel, 92(1) (2012) 162-170. [17] I. Ye, C. Ryu, Numerical modeling of slag flow and heat transfer on the wall of an entrained coal gasifier, Fuel, 150 (2015) 64-74. [18] I. Ye, C. Ryu, J.H. Koo, Influence of critical viscosity and its temperature on the slag behavior on the wall of an entrained coal gasifier, Applied Thermal Engineering, 87 (2015) 175-184. [19] M. Otaka, H. Watanabe, S. Hara, Numerical simulation of molten-slag flow in coal gasifier, in: 2005 ASME Power Conference, April 5, 2005 - April 7, 2005, American Society of Mechanical Engineers, Chicago, IL, United states, 2005, pp. 1471-1476. [20] D. Wang, X. Ling, H. Peng, Theoretical analysis of free-surface film flow on the rotary granulating disk in waste heat recovery process of molten slag, Applied Thermal Engineering, 63(1) (2014) 387-395. [21] B. Zhao, S.P. Vanka, B.G. Thomas, Numerical study of flow and heat transfer in a molten flux layer, International Journal of Heat and Fluid Flow, 26(1) (2005) 105-118. [22] S. Liu, Y. Hao, Numerical study on slag flow in an entrained-flow gasifier, in: ASME International Mechanical Engineering Congress and Exposition, IMECE 2007, November 11, 2007 - November 15, 2007, American Society of Mechanical Engineers, Seattle, WA, United states, 2008, pp. 793-800. [23] C. Wieland, B. Kreutzkam, G. Balan, H. Spliethoff, Evaluation, comparison and validation of deposition criteria for numerical simulation of slagging, Applied Energy, 93 (2012) 184-192. [24] M. Risberg, P. Carlsson, R. Gebart, Numerical modeling of a 500 kW air-blown cyclone gasifier, Applied Thermal Engineering, 90 (2015) 694-702. [25] A.V. Ryzhakov, Role of slag coating in the dynamics of transient processes of a lined furnace chamber, Teploenergetika, (3) (1973) 50-53. [26] X. Li, G. Li, Z. Cao, S. Xu, Research on flow characteristics of slag film in a slag tapping gasifier, Energy & Fuels, 24(9) (2010) 5109-5115. [27] C. Yan, B. Lin, E. Chen, Liquid slag film thickness solution model and heat transfer analysis for liquid slag-removal burner, Boiler Technology, 32(7) (2001) 7-9 (in Chinese). 61
[28] D. Che, Boilers – Theory, Design and Operation, 1st ed., Xi'an Jiaotong University Press, Xi'an, 2008. [29] ВТИ, ЦКТИ, Thermal Calculation of Boiler Units – Standard Method, Energiya Press, Moscow, 1973 (in Russian).
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Figure Captions Fig. 1. Cyclone barrel pin stud and refractory section Fig. 2. Multilayer wall model of heat transfer in the cyclone barrel Fig. 3. Schematic diagram of liquid slag film along the perimeter of the cyclone barrel Fig. 4. Comparison of the cyclone barrel exit gas temperatures obtained through the existing test data (or CFD) and method of the present study Fig. 5. Load characteristics of the cyclone barrel: (a) fuel consumption rate; (b) averaged heat flux; (c) surface temperature distributions of different layers; (d) thicknesses of liquid and solid slag layers; ( D = 2.4 m, L = 9.6 m, rl = 20 mm, = 1.1, tph = 400 °C and qF0 = 20000 kw/m2) Fig. 6. Effect of barrel diameter on the adaptability ( L /D = 4.0, rl = 20 mm, = 1.1, tph = 400 °C and qF0 = 20000 kw/m2) Fig. 7. Effect of thickness of refractory lining on the adaptability ( D = 2.4 m, L = 9.6 m, = 1.1, tph = 400 °C and qF0 = 20000 kw/m2)
Fig. 8. Effect of excess air ratio on the adaptability ( D = 2.4 m, L = 9.6 m, rl = 20 mm, tph = 400 °C and qF0 = 20000 kw/m2) Fig. 9. Effect of temperature of preheated air on the adaptability ( D = 2.4 m, L = 9.6 m, rl = 20 mm, = 1.1 and qF0 = 20000 kw/m2) Fig. 10. Effect of ash content on the adaptability ( D = 2.4 m, L = 9.6 m, rl = 20 mm, = 1.1, tph = 400 °C and qF0 = 20000 kw/m2)
Fig. 11. Effect of coal slag temperature of critical viscosity on the adaptability ( D = 2.4 m, L = 9.6 m, rl = 20 mm, = 1.1, tph = 400 °C and qF0 = 20000 kw/m2)
63
Figures:
Fig. 1
Fig. 2
Fig. 3
64
Fig. 4
Fig. 5
65
Fig. 6
Fig. 7
66
Fig. 8
Fig. 9
67
Fig. 10
Fig. 11
68
Table Captions Table 1 Coal property analysis of selected fuel (wt %) Table 2 Main design parameters and operating conditions of a cyclone barrel Table 3 Ultimate analyses of four studied bituminous coals (wt %)
Tables: Table 1 Coal property analysis of selected fuel (wt %) Car
Har
Oar
Nar
Sar
Aar
Mar
Vdaf
Qnet,ar
t0
0
v
%
%
%
%
%
%
%
%
kJ/kg
°C
Pa·s
1/°C
58.86
3.36
6.98
0.79
0.63
19.78
9.60
32.31
22440
1360
14
0.015
Table 2 Main design parameters and operating conditions of a cyclone barrel Design Parameters
Operating Conditions
Parameter
Symbol /(Unit)
Value
Parameter
Symbol /(Unit)
Value
Barrel diameter
D /(m)
2.4
Excess air ratio
/(-)
1.1
tph /(°C)
400
w2 /(m/s)
90
pg /(MPa)
10
tg /(°C)
311.0
Temperature of Barrel length
L /(m)
9.6 preheated air Secondary air
Barrel volume
V /(m3)
43.4 velocity
Barrel inside
Saturated working 2
As,i /(m )
81.4
surface area
fluid pressure
Thickness of
Saturated working
rl /(mm)
20
refractory lining
fluid temperature
69
Table 3 Ultimate analyses of four studied bituminous coals (wt %) Car
Har
Oar
Nar
Sar
Aar
Mar
Vdaf
Qnet,ar /(kJ/kg)
69.44
3.99
6.76
0.73
1.40
9.68
8.00
30.70
26820
58.86
3.36
6.98
0.79
0.63
19.78
9.60
32.31
22440
50.50
3.50
6.00
1.00
1.00
30.00
8.00
25.50
20010
40.73
3.42
5.21
0.98
0.77
39.69
9.20
25.00
16800
70
Highlights:
An appropriate method was proposed to evaluate the adaptability of cyclone barrel.
A calculation model and a judgment criterion were developed in the method.
There is a load range for the cyclone barrel to meet the judgment criterion.
Effects of various factors on the adaptability were investigated systematically.
71
S1359-4311(17)32467-5 http://dx.doi.org/10.1016/j.applthermaleng.2017.04.043 ATE 10192
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
24 May 2016 30 March 2017 12 April 2017
Please cite this article as: S. Wu, L. Deng, C. Wang, C. Tang, Y. Bu, D. Che, A Study on the Adaptability of Cyclone Barrel to Slag Film in a Cyclone-fired Boiler, Applied Thermal Engineering (2017), doi: http://dx.doi.org/10.1016/ j.applthermaleng.2017.04.043
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A Study on the Adaptability of Cyclone Barrel to Slag Film in a Cyclone-fired Boiler
Song Wu, Lei Deng, Chang’an Wang, Chunli Tang, Yufan Bu, Defu Che
State Key Laboratory of Multiphase Flow in Power Engineering, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China
To whom correspondence should be addressed. Tel.: +86-029-82665185; fax: +86-029-82668703.
E-mail address: [email protected] (D. Che).
1
Abstract: The behavior of slag film inside the cyclone barrel is closely related to its design and operation. In this paper, a multilayer wall model considering the heat transfer from flame to working fluid in the water-cooled tubes was proposed and a judgment criterion was established to evaluate the adaptability of cyclone barrel to slag film. Furthermore, parametric investigation was carried out systematically to analyze the influencing factors of the adaptability, including design parameters (barrel diameter, thickness of refractory lining), operating conditions (excess air ratio, temperature of preheated air) and coal properties (ash content, coal slag temperature of critical viscosity). The results indicate that the molten slag can plug the slag tap at low loads while erode and corrode the refractory lining severely at high loads for a cyclone barrel. There is a load range within which the cyclone barrel operates safely and steadily. The thickness of refractory lining and coal slag temperature of critical viscosity are the key factors affecting the adaptability significantly. The adaptability descends as the thickness of refractory lining increases and a lower coal slag temperature of critical viscosity leads to a decrease in the adaptability. The effects of other factors are relatively small. Especially for barrel diameter, its impact can be disregarded. The conclusions in the present study are helpful to the optimization design and safety operation of cyclone barrels. Keywords: cyclone-fired boiler, cyclone barrel, slag film, heat transfer, thermal performance, adaptability
2
1. Introduction The cyclone furnace concept was first proposed by the Babcock & Wilcox Company in the 1940s [1]. In general, a cyclone furnace boiler consists of one or several cyclone barrels in which pulverized coal burns strongly at high temperatures (> 1650 °C) and the coal ash is melted. With the swirling motion of the secondary air, a molten slag layer forms and coats the inside surface of the cyclone barrel. The slag is discharged through the slag tap and enters the slag tank. Cyclone furnaces are widely used for utility boilers, industrial boilers or kilns. Especially, they are extremely suitable for firing coals with low ash melting points, anthracites and high-sodium coals [2]. However, due to the high combustion temperature, relatively high NOx emissions were observed in cyclone-fired boilers, which made the cyclone units less popular. With the development of combustion technology, considerable reductions in NOx levels have been demonstrated in some cyclone boilers using air staging or reburning in recent years [1, 3-7]. As a result, the cyclone-fired boilers gain the comparable competitiveness with the pulverized coal boilers. Both the plugging of slag tap and the excessive erosion/corrosion of refractory lining are the serious problems for cyclone furnaces, which can result in the equipment damages and unit outages [8, 9]. They are bound up with the formation, flow and heat transfer of slag film inside the cyclone barrel. The adaptability of cyclone barrel to slag film represents the ability of cyclone barrel to operate well without the occurrence of plugging and excessive erosion/corrosion. Obviously, the adaptability is determined by the behavior of slag film.
34
Many research efforts have been devoted to the behavior of slag film [10-25]. Some researchers [10-13] experimentally simulated the capture efficiencies and flow characteristics of slag by the use of the imitations of molten slag such as water spray, paraffin and syrup. But these studies were all performed in cold model experiments and couldn’t reveal the actual slag film behavior with heat transfer. Cheng et al. [14] detected the deposition and flow of slag in a lab-scale entrained-flow gasifier visually with the help of heat resisting endoscope and digital image processing technology. The results suggested that slag flow velocity has a positive correlation with slag surface temperature and reaches about 0.0026 to 0.003 m/s under the experimental conditions. Moreover, many investigators [15-24] employed the numerical simulation methods to evaluate the behavior of slag film. Seggiani [15] proposed a slag building model in a Prenflo coal gasifier to describe the time varying behavior of the slag layer. The results demonstrated that the model could predict the slag behavior reasonably. A steady-state model considering the particle collision and capture was presented in the work of Yong et al. [16]. Their research showed that particle momentum has a significant effect on the build-up of slag layer. Ye et al. [17, 18] developed a new numerical model to overcome the limitations of existing models and examined the impacts of critical viscosity and its temperature on the behavior of slag film. There were also some researchers utilizing the commercial computational fluid dynamics codes to predict the formation, flow and heat transfer of slag film, and satisfactory results were obtained [21-24]. These models and conclusions are useful to gasifiers but can not be extrapolated to cyclone boilers directly because of lacking sufficient
35
validation. In addition, Ryzhakov [25] investigated the effect of slag film on transient processes in the cyclone barrel and reported that the self-regulation in the thickness of slag layer plays an important role in the response of thermal performance when the operating conditions change. These efforts are valuable for understanding the behavior of slag film. However, there is little literature available regarding how slag film works when the plugging or excessive erosion/corrosion occurs in a cyclone furnace. Meanwhile, the adaptability of cyclone barrel to slag film, which is of great significance for the design and operation of cyclone barrels, is still unclear and insufficiently understood. Therefore, it’s necessary to seek a way to analyze and assess the adaptability to guide engineering practice. Furthermore, since the adaptability depends on a number of factors, such as design parameters, operating conditions and coal properties, it’s more desirable to study the influences of these factors systematically. The purpose of this study is to propose an appropriate method to predict the adaptability of cyclone barrel to slag film. To do so, a multilayer wall model of heat transfer was developed to calculate the thermal performance of slag film, and a judgment criterion was presented to evaluate whether the plugging and excessive erosion/corrosion occur or not. Then, this method was employed to analyze a cyclone barrel in detail. Moreover, the effects of various factors on the adaptability were also investigated, including barrel diameter, thickness of refractory lining, excess air ratio, temperature of preheated air, ash content and coal slag temperature of critical viscosity.
36
2. Method of Evaluation 2.1. Multilayer wall model of heat transfer As shown in Fig. 1, the cyclone barrel is of water-cooled tube construction. Short pin studs are welded on the outside surface of the tubes in a very dense pattern and a refractory material is installed around the pin studs. In this way a refractory lining is generated. During the operation process, the refractory lining needs to undergo the thermal stress of high temperature and the erosion and corrosion caused by the molten slag. Though the refractory lining faces an extreme working environment, the solid slag layer is able to protect it from the erosion and corrosion. This protection mechanism is called “using slag to prevent slag”.
Fig. 1. Cyclone barrel pin stud and refractory section
In the actual process, the slag layer on the refractory lining can be subdivided into a liquid slag layer, a plastic flow layer and a solid slag layer. The interface between the liquid slag layer and the plastic flow layer is determined by the coal slag temperature of critical viscosity ( t0 ). The molten slag could be considered as a Newtonian fluid when its temperature is above t0 , while as a plastic fluid if the temperature is below
t0 . Generally, the movement of the liquid slag layer is slight [14]. The corresponding Reynolds number is < 0.03, meaning that the flow pattern of the liquid slag layer belongs to laminar flow. Thus, conduction along the normal direction basically controls the heat transfer in the liquid slag layer. Moreover, compared with the barrel diameter (inside diameter of cyclone barrel), the average thickness of liquid slag layer
37
is so thin that the process of heat transfer can be treated as a heat conduction problem in the plane wall [26]. The slag flow in the plastic region is neglected due to its high viscosity. The plastic flow layer and solid slag layer can be considered together as one layer here, still referred to as solid slag layer. The refractory lining is composed of metal pin studs and refractory which have different thermal conductivities. In the present study, an averaged heat flux was employed to describe the heat conduction in the refractory lining with a composite thermal conductivity adopted. The assumption of plane wall is also suitable for the solid slag layer and refractory lining. Here, a multilayer wall model of heat transfer is proposed, as illustrated in Fig. 2. The whole process of heat transfer from flame to working fluid is taken into account in the model. When the coal particles are burning, heat is transferred from the high temperature flame to the fireside surface of liquid slag layer by radiation and convection. Then, the heat flows through the liquid slag layer, solid slag layer, refractory lining, and metal tube wall in turn by conduction and reaches the inside surface of the metal tube. Finally, the working fluid absorbs the heat by convection. The scale deposit is negligible since the rigorous water treatment and boiling out previous to operation are always performed. Additionally, the following assumptions are applied in the present model: (1) All the calculation and analysis are carried out in steady-state conditions. (2) The heat transfer is normal to the plane wall. (3) The surface temperatures of various layers are uniform. (4) The temperature profiles in both liquid and solid slag layers are linear.
38
(5) The thicknesses of both liquid and solid slag layers are considered uniform.
Fig. 2. Multilayer wall model of heat transfer in the cyclone barrel
Actually, the slag film thickness is not uniform along the perimeter of the cyclone barrel because of gravity. As shown in Fig. 3, for the liquid slag film on the left side, the tangential component of gravity is in the direction of flue gas flow, which can accelerate the circumferential flow of liquid slag. Conversely, the tangential component of gravity is in the opposite direction of flue gas flow and the circumferential flow of liquid slag is inhibited on the right side. Under such a difference, the liquid slag of left side moves faster than that of right side in the circumferential direction. As a result, the slag film on the left side is thinner than that on the right side [27]. Nevertheless, the difference in the slag film thickness is not significant due to the scouring action of fast rotating flame flow on the surface of slag film. The scouring action can form a uniformly distributed tangential force driving the rotating flow of slag film. The tangential force includes the viscous force of gas phases and the impact force of particles. The circumferential behavior of slag film is the result of the joint action of the tangential force and gravity. The tangential force is much larger than the gravity and plays a major role in the circumferential flow of slag film. Therefore, the gravity can only affect the circumferential distribution of the slag film thickness slightly. Furthermore, with the dip angle ( ) increasing, the gravity component in the cross section of the cyclone barrel is reduced. Correspondingly, the effect of gravity on the slag film thickness along the circumferential direction
39
weakens. Specially, when the cyclone barrel is arranged perpendicularly, i.e., the vertical cyclone furnace, the gravity will lose its influence power on the circumferential behavior of slag film completely. In summary, the slag film thickness along the perimeter of the cyclone barrel can be regarded as uniform distribution approximately without significant differences introduced, and the assumption of uniform slag film thickness is reasonable in the present study.
Fig. 3. Schematic diagram of liquid slag film along the perimeter of the cyclone barrel
2.2. Thermal performance calculation In accordance with the model in Fig. 2, heat transfer in the multilayer wall of cyclone barrel can be expressed by: qx
tzm t0 t0 trl trl two two twi twi tg Rzm,l Rzm,s Rrl Rw Rc,g
(1)
where qx is the averaged heat flux in the multilayer wall, tzm , trl , two , t wi and tg are the temperatures of liquid slag layer fireside surface, refractory lining fireside surface, water-cooled tube outside surface, water-cooled tube inside surface and saturated working fluid respectively, Rzm,l , Rzm,s , Rrl , Rw and Rc,g are the heat resistances of liquid slag layer, solid slag layer, refractory lining, metal tube wall and convection between working fluid and tube inside surface respectively. The flame and the cyclone barrel enclosure can be considered to be two infinite parallel plates. According to the Stefan-Boltzmann law, heat transfer of radiation in the cyclone barrel is formulated as:
40
Qr =
0 aK H r Bj
4 (Ty4 Tzm )
(2)
where Qr is the quantity of radiant heat transfer, 0 is the Stefan-Boltzmann constant, aK is the system emissivity of cyclone barrel, H r is the area of effective radiative heating surface, Ty is the effective average temperature of flame in K, Tzm is the temperature of liquid slag layer fireside surface in K, and Bj is the calculation fuel consumption rate for cyclone barrel. Dividing by B j means the heat transfer is computed based on the calculation fuel consumption of unit mass. The system emissivity of cyclone barrel is defined as: aK
1 1 1 x( 1) aw ay
(3)
where aw and ay are the emissivities of furnace wall and flame respectively, and
x is the averaged angular factor of cyclone barrel, which is calculated as [28]: x
x F F i
bi
(4)
bi
where xi and Fbi are the angular factor and furnace wall area of zone i respectively. For the cyclone barrel, the angular factor of barrel wall is 1, and those of burner and secondary air ports are taken as 0. The temperature of flue gas varies along the cyclone barrel length. In this study, a characteristic temperature, i.e., the effective average temperature of flame, is employed to describe the radiant heat transfer in the cyclone barrel. The effective average temperature of flame is valued as:
Ty 0.925 TaTg,e
41
(5)
where Ta is the adiabatic flame temperature in K, and Tg,e is the cyclone barrel exit gas temperature in K. Detailed information on the explanation, derivation and validation of Eq. (5) is provided in the Part 1 of the Supplementary Materials. Owing to the strong rotation motion inside the cyclone barrel, the convection action should be considered. Heat transfer of convection in the cyclone barrel is represented by: Qc
U c H r (Ty Tzm ) Bj
(6)
where Qc is the quantity of convective heat transfer, and U c is the convective heat transfer coefficient. For the convective heat transfer process in the cyclone barrel, it is hard to measure or describe it separately. In order to predict the heat transfer by convection accurately, a modeling test was conducted to seek the convective heat transfer correlation [29]. According to the similarity theory, the Reynolds number was selected as the scaling criterion. Keeping the Reynolds number identical, the similar flow field can be created in the modeling test setup. The test conditions were designed in accordance with the range of the actual operating conditions for cyclone barrels. Through the statistical analyses on a great number of experimental data, the practical correlation is derived as:
Nuc 0.0074Rec
(7)
where Nuc is the Nusselt number, and Rec is the Reynolds number. The barrel diameter is used as the feature size in both of the criterion numbers. They are computed in Eq. (8) and Eq. (9): 42
Nuc
Uc D
(8)
Rec
w2 D
(9)
y
y
where D is the barrel diameter, y is the thermal conductivity of flue gas, w2 is the secondary air velocity, and y is the kinematic viscosity of flue gas. To get tzm or Tzm , an implicit equation is given as Eq. (10). The standard method for thermal calculation of former Soviet Union provides a nomogram to solve it [29]. More information on the solution procedure of the nomogram is provided in the Part 2 of the Supplementary Materials.
tzm t0 f (Ty , t0 , v , A, w2 ) Ty
(10)
where v is the viscosity property factor of slag, and A is the parameter reflecting the thickness of liquid slag layer. They are defined in Eq. (11) and Eq. (12): ln
v
0
t t0
A 0.467 3
G0 0s u s2 ps
(11)
(12)
where 0 is the critical viscosity of slag, is the viscosity of slag at the temperature of t , G0 is the ash mass flow rate entering cyclone barrel, s is the slag capture efficiency, u is the cyclone barrel perimeter wetted by slag, s is the density of slag, and ps is the parameter reflecting the dynamic impact of flue gas on slag film. The slag capture efficiency represents the mass fraction of bottom slag in the total ash of fuel. In cyclone furnaces, 70% to 85% of the fuel ash is captured by
43
the barrel wall and the remainder is carried by flue gas to enter the convection pass [1, 29]. Therefore, s is in the range of 0.7 - 0.85 and generally selected as 0.8 in the design or analysis. Since the inside surface of the cyclone barrel is completely coated by a molten slag layer under normal operation, the cyclone barrel perimeter wetted by slag is virtually the inside perimeter of the cyclone barrel ( D ). Furthermore, heat balance in the cyclone barrel is written as:
Qr Qc (Qa I g,e )
(13)
where is the heat retention factor, Qa is the available heat entering cyclone barrel, and I g,e is the cyclone barrel exit gas enthalpy. The heat retention factor represents the fraction of the heat transferred to working fluid through the heating surface in the heat released by flue gas ( 1.0 ), that is to say, most part of the heat released by flue gas is transferred to working fluid, and the remainder is lost to the environment through the outer surface of the boiler setting. is calculated as follows:
1
q5 b q5
(14)
where q5 is the radiation and convection heat loss and b is the thermal efficiency of a boiler. By coupling with the heat transfer in the multilayer wall, qx can be given as:
qx
Bj (Qr Qc ) Hr
(15)
In addition to the above equations, Eq. (16) is supplemented to solve Eq. (1):
R
44
(16)
where R is the heat resistance, is the layer thickness, and is the thermal conductivity. Eq. (16) works within all the layers, including liquid slag layer, solid slag layer, refractory lining and metal tube wall. Especially, the composite thermal conductivity of refractory lining ( rl ) can be calculated as:
rl
r m
f r (1 f )m
(17)
where r and m are the thermal conductivities of refractory and metal pin studs respectively, and f is the arrangement density of pin studs. In the present study, the refractory material is of SiC and the pin studs are of 12Cr1MoV alloy steel. According to the operating temperature ranges of the refractory and metal pin studs, their thermal conductivities are 5.814 and 30.232 W/(m·K) respectively, and f is a structure parameter designed as 0.2. With all the equations solved simultaneously, the thermal performances of slag film under given conditions can be derived, such as boundary temperatures of each layer, slag layer thicknesses, averaged heat flux and so on, which will be of help to analyze the adaptability of cyclone barrel to slag film. 2.3. Judgment criterion of adaptability Based on the obtained thermal performances, a judgment criterion of adaptability is presented to evaluate the operation state of cyclone barrel. When the operating conditions, load or fuel changes, the t0 interface in Fig. 2 will move accordingly. In particular, the solid slag layer disappears with the t0 interface moving left and arriving at the fireside surface of refractory lining, which makes the cyclone barrel lose the protection mechanism. In other words, when the thickness of solid slag layer 45
( zm,s ) becomes zero, the molten slag would erode and corrode the refractory lining severely. Furthermore, considering the requirement of slag tapping, the cyclone barrel exit gas temperature ( g,e ) should be always maintained higher than t0 to prevent the slag from plugging the tap. As a result, the judgment criterion is given as follows:
g,e t0 & zm,s 0
(18)
2.4. Method validation In order to verify the validity of the method used in this study, a comparison of the cyclone barrel exit gas temperatures obtained through the existing test data, computational fluid dynamics (CFD) and method of the present study was carried out, as shown in Fig. 4. Three cyclone barrels with different capacities are available to provide the test data and the numerical calculation data. The CFD study was performed in a vertical cyclone barrel with a heat release rate of 61.3 MW, which can give the numerical results of the temperature distribution along the cyclone barrel length (see Part 1 of the Supplementary Materials). According to the comparative results, the maximum deviation of the method used in the present study is 29.3 °C for the cyclone barrel exit gas temperature. Such an agreement is quite satisfactory from an engineering perspective. Since the cyclone barrel exit gas temperature is determined by the heat transfer process in the cyclone barrel, the agreement in the cyclone barrel exit gas temperature demonstrates the validity in the prediction of the heat transfer in the cyclone barrel, and the method of the present study is creditable and reasonable.
46
Fig. 4. Comparison of the cyclone barrel exit gas temperatures obtained through the existing test data (or CFD) and method of the present study
Above CFD simulation was carried out by using the commercial software Fluent. The conservation equations of mass, momentum, and energy are solved by the SIMPLE algorithm. In order to model the strong swirling flow in the cyclone barrel properly, the standard k-ε model is chosen to consider the turbulence flow of gases. Particles are modelled with the discrete phase (DPM) model, and particle trajectories are calculated through the Lagrangian approach. Radiation heat transfer is described by the discrete ordinate (DO) model. The absorption coefficient is obtained by the weighted-sum-of-gray-gas (WSGG) model. The probability density function (PDF) is employed to simulate the homogeneous combustion. In addition, the single-step model
is
applied
to
describe
the
coal
devolatilization
and
the
kinetics/diffusion-limited model is used for the char combustion. These models are all widely used in the furnace simulation, which ensures the reliability of the numerical results. 3. Results and Discussion 3.1. Load characteristics According to the above model and formulas, the thermal performances of a cyclone barrel under various heat release rates per unit barrel cross-sectional area ( qF ) within a range of 4000 - 40000 kW/m2 are calculated. The coal property analysis of selected fuel is shown in Table 1. The main design parameters and operating conditions of the cyclone barrel are listed in Table 2.
47
Table 1 Coal property analysis of selected fuel (wt %)
Table 2 Main design parameters and operating conditions of a cyclone barrel
The load characteristics of the cyclone barrel are illustrated in Fig. 5. The fuel consumption rate for cyclone barrel ( B ) is proportional to qF in Fig. 5(a). Because the operation load depends on B , qF can be utilized to represent the level of operation load. As shown in Fig. 5(b), qx increases with qF rising while the increase range reduces, indicating that the heat absorbed by the fireside surface of liquid slag layer from flame is raised as the operation load rises. The increase of flame temperature leads to the rise in qx . The reduction of the increase range is attributed to the decrease in the heat transfer quantity which is computed according to the calculation fuel consumption of unit mass. Variations in the surface temperatures of different layers are illustrated in Fig. 5(c). With the operation load rising, there is a slight increase in t wi . Because the boiling heat transfer occurs in the water-cooled tubes, Rc,g is quite small and t wi is close to tg . Moreover, two presents a similar tendency with t wi due to the large enough thermal conductivity of metal. As qF rises, trl increases and a decrease in zm,s is observed in Fig. 5(d), which shows a good agreement with the research results of Ryzhakov [25]. Especially, when qF rises closely to 40000 kW/m2, trl is equal to t0 and zm,s becomes zero simultaneously. The cyclone barrel can’t meet the judgment criterion of adaptability under this condition. Thus, the maximum operation load for safety is derived. g,e
48
reduces as qF decreases. With qF falling closely to 4000 kW/m2, g,e equals t0 and the slag tap may become plugged. Therefore, the minimum operation load for safety can be obtained. In addition, it can be seen from Fig. 5(c) and Fig. 5(d) that tzm and the thickness of liquid slag layer ( zm,l ) have a slight variation with the changing operation load owing to the self-regulation of zm,s [18].
Fig. 5. Load characteristics of the cyclone barrel: (a) fuel consumption rate; (b) averaged heat flux; (c) surface temperature distributions of different layers; (d) thicknesses of liquid and solid slag layers; ( D = 2.4 m, L = 9.6 m, rl = 20 mm, = 1.1, tph = 400 °C and qF0 = 20000 kw/m2)
3.2. Effects of design parameters Fig. 6 compares the adaptabilities of various cyclone barrels with a range of 1.6 3.2 m in barrel diameter ( D ). The length/diameter ratio ( L /D ) is chosen as 4.0 and the values of other parameters are the same as those listed in Tables 1 and 2. Fig. 6 indicates that the thermal performances of the cyclone barrels exhibit the similar variation tendencies as qF changes. For a given qF , qx increases with the increasing D . An increase in qx results in a rise in trl and a reduction in g,e .
zm,s decreases as D increases in Fig. 6(d). When qF is increased, the larger D is, the sooner zm,s becomes zero. Therefore, the load range under which the cyclone barrel can maintain security and stability is narrowed as D rises. In other words, the adaptability of cyclone barrel to slag film reduces with D increasing. Noticeably, the variations in the thermal performances are quite small for a given qF , e.g., the maximum temperature difference in g,e is < 30 °C during the whole load range. The
49
thermal performances at low loads change little with D . Thus, the effect of barrel diameter on the adaptability is considered unremarkable.
Fig. 6. Effect of barrel diameter on the adaptability: (a) averaged heat flux; (b) temperature of refractory lining fireside surface; (c) cyclone barrel exit gas temperature; (d) thickness of solid slag layer; ( L /D = 4.0, rl = 20 mm, = 1.1, tph = 400 °C and qF0 = 20000 kw/m2)
Fig. 7 shows the effect of thickness of refractory lining ( rl ) on the adaptability. The composite thermal conductivity of refractory lining is valued as 6.934 W/(m·K) according to Eq. (17). Within a range of 10 - 30 mm in rl , the other parameters are shown in Tables 1 and 2. In practice, the solid slag layer can be considered as the extension of refractory lining. For a given operation condition, if the overall heat resistance of refractory lining and solid slag layer is kept constant, the cyclone barrel will work invariably regardless of their thicknesses. The variation in rl is offset by the self-regulation of zm,s . As rl increases, zm,s decreases due to the left movement of the t0 interface (see Fig. 2) and vice versa. Thus, rl mainly affects the solid slag layer while the liquid slag layer is hardly affected. Accordingly, the heat transfer between flame and fireside surface of liquid slag layer is also uninfluenced, as illustrated in Fig. 7(a) and Fig. 7(c). trl increases with rl rising when a qF is given in Fig. 7(b). It can be seen from Fig. 7(d) that zm,s decreases with the increasing rl , demonstrating the adaptability of cyclone barrel declines. Moreover,
rl has a significant effect on the adaptability. When rl is increased from 20 to 25 mm, the load range for safety is cut by about half ( q1 ), and q2 is less than q1 .
50
As discussed above, the refractory lining can not be designed too thick or thin. The adaptability will not meet the requirement of load regulation if the refractory lining is too thick, while the wear resistance is not enough if too thin.
Fig. 7. Effect of thickness of refractory lining on the adaptability: (a) averaged heat flux; (b) temperature of refractory lining fireside surface; (c) cyclone barrel exit gas temperature; (d) thickness of solid slag layer; ( D = 2.4 m, L = 9.6 m, = 1.1, tph = 400 °C and qF0 = 20000 kw/m2)
3.3. Effects of operating conditions Fig. 8 presents the effect of excess air ratio ( ) on the adaptability. With varying from 1.05 to 1.2, the other parameters are listed in Tables 1 and 2. Fig. 8(a) indicates that qx falls as rises for a given qF . This is because the temperature of preheated air is far lower than that of the combustion product. An increase in the amount of preheated air leads to a decrease in the flame temperature, which reduces the quantity of heat transfer in the cyclone barrel. In accordance with the variation of
qx , trl falls with increasing in Fig. 8(b). A transition point appears at the qF of 10600 kW/m2 in Fig. 8(c). For qF below 10600 kW/m2, g,e increases as rises. Inversely, g,e declines as rises when qF is above 10600 kW/m2. With an increase of , a decrease in the quantity of heat transfer results in an increase in g,e , while the blend of the increased preheated air and high temperature combustion product can cause a reduction simultaneously. The actual g,e is determined by the comprehensive result of these two effects. The heat transfer plays a dominant role at
51
low loads while the blend of different enthalpies possesses a leading position at high loads. As shown in Fig. 8(d), zm,s increases with
rising, meaning the
adaptability of cyclone barrel rises. In addition, the cyclone barrel utilizing air staging technology is also analyzed. With an excess air ratio of 0.8, partial gasification occurs in the cyclone barrel during the combustion of pulverized coal. The curves for of 0.8 are quite close to those for of 1.2 owing to the accordance in qx . The incomplete combustion of fuel reduces the flame temperature, leading to a decrease in
qx compared with other situations.
Fig. 8. Effect of excess air ratio on the adaptability: (a) averaged heat flux; (b) temperature of refractory lining fireside surface; (c) cyclone barrel exit gas temperature; (d) thickness of solid slag layer; ( D = 2.4 m, L = 9.6 m, rl = 20 mm, tph = 400 °C and qF0 = 20000 kw/m2)
The effect of temperature of preheated air ( tph ) on the adaptability is shown in Fig. 9. Within a range of 280 - 440 °C in tph , the other parameters are listed in Tables 1 and 2. As tph rises, the flame temperature increases and the heat transfer in the cyclone furnace is enhanced. As a result, qx rises with the increasing tph for a given qF in Fig. 9(a). Correspondingly, trl increases with tph rising in Fig. 9(b). The impact of tph on g,e is similar to that of . The enhanced heat transfer results in a decrease in g,e , while the increased enthalpy of air leads to an increase simultaneously. Since the blend of different enthalpies possesses a leading position at high loads, g,e rises as tph increases in Fig. 9(c). It can be seen from Fig. 9(d) that
zm,s reduces with the increasing tph and the adaptability of cyclone barrel decreases
52
accordingly.
Fig. 9. Effect of temperature of preheated air on the adaptability: (a) averaged heat flux; (b) temperature of refractory lining fireside surface; (c) cyclone barrel exit gas temperature; (d) thickness of solid slag layer; ( D = 2.4 m, L = 9.6 m, rl = 20 mm, = 1.1 and qF0 = 20000 kw/m2)
3.4. Effects of coal properties Fig. 10 illustrates the effect of ash content ( Aar ) on the adaptability. Four bituminous coals are investigated and the ultimate analyses are listed in Table 3. For the studied coals, the ash contents are 9.68%, 19.78%, 30.00% and 39.69% respectively and the maximum difference between moisture contents is < 2%. The properties of coal slag are shown in Table 1 and the other parameters are listed in Table 2. When a qF is given, the more Aar is, the more the yield of molten slag is. An increase in the yield of molten slag can thicken the liquid slag layer, resulting in a rise of tzm . Therefore, the heat transfer from the high temperature flame to the fireside surface of liquid slag layer weakens and qx decreases, as shown in Fig. 10(a). Accordingly, trl falls as Aar increases in Fig. 10(b). A transition point is observed at the qF of 23000 kW/m2 in Fig. 10(c). When qF is below 23000 kW/m2,
g,e rises with the increasing Aar . However, g,e falls with the increase of Aar for qF above 23000 kW/m2. As Aar increases, a decrease in qx causes a rise in g,e , while a decrease of adiabatic flame temperature can lead to a reduction simultaneously. The relative strength of the two effects is able to account for the
53
actual g,e . As illustrated in Fig. 10(d), with Aar increasing, zm,s rises and the adaptability of cyclone barrel increases.
Fig. 10. Effect of ash content on the adaptability: (a) averaged heat flux; (b) temperature of refractory lining fireside surface; (c) cyclone barrel exit gas temperature; (d) thickness of solid slag layer; ( D = 2.4 m, L = 9.6 m, rl = 20 mm, = 1.1, tph = 400 °C and qF0 = 20000 kw/m2) Table 3 Ultimate analyses of four studied bituminous coals (wt %)
Fig. 11 describes the effect of coal slag temperature of critical viscosity ( t0 ) on the adaptability. Within a range of 1150 - 1450 °C in t0 , the other parameters are listed in Tables 1 and 2. qx reduces with t0 rising for a given qF in Fig. 11(a). The higher
t0 is, the higher tzm is. Therefore, the temperature difference of heat transfer from the high temperature flame to the fireside surface of liquid slag layer decreases as t0 increases, and the quantity of heat transfer reduces accordingly. In accordance with the variation of qx , trl falls with the increasing t0 in Fig. 11(b). A decrease in qx can lead to an increase in g,e . Therefore, the higher t0 is, the higher g,e is. According to the judgment criterion of adaptability for slag tapping, the minimum operation load for safety changes slightly with t0 , as shown in Fig. 11(c). Furthermore, the flame temperature will be increased when the coals with high ash melting points are fired, admitting the cyclone furnaces to deal with more coals effectively. zm,s rises with the increasing t0 in Fig. 11(d) [15, 18], indicating the adaptability of cyclone barrel increases. It is worth noticing that the effect of t0 is
54
quite remarkable. When t0 rises from 1150 to 1250 °C, the load range for safety is extended by about 80% ( q1 ), and q2 is more than q1 . If the adaptability is too small to meet the operation requirement for the coals with lower ash melting points, reducing thickness of refractory lining or increasing the arrangement density of pin studs will be applied for improvement.
Fig. 11. Effect of coal slag temperature of critical viscosity on the adaptability: (a) averaged heat flux; (b) temperature of refractory lining fireside surface; (c) cyclone barrel exit gas temperature; (d) thickness of solid slag layer; ( D = 2.4 m, L = 9.6 m, rl = 20 mm, = 1.1, tph = 400 °C and
qF0 = 20000 kw/m2)
4. Conclusions In this study, a multilayer wall model of heat transfer from flame to working fluid in the cyclone barrel was proposed and a judgment criterion was given to evaluate the adaptability of cyclone barrel to slag film. Then, the load characteristics and various influencing factors of the adaptability were probed in detail. The following conclusions can be drawn: The cyclone barrel exit gas temperature declines as the operation load falls, and the thickness of solid slag layer decreases as the operation load rises. There is a load range for the cyclone barrel to meet the judgment criterion of adaptability. The adaptability is affected by the thickness of refractory lining and coal slag temperature of critical viscosity significantly, while the effects of other factors are relatively small. Especially, the impact of barrel diameter can be neglected. The adaptability decreases with the thickness of refractory lining increasing, and the
55
influence on heat transfer can be offset by the self-regulation in the thickness of solid slag layer. A lower coal slag temperature of critical viscosity also results in a reduction of the adaptability. Since the adaptability of cyclone barrel to slag film depends on many influential factors simultaneously, further research on multiparameter combined optimization is needed. Acknowledgements The authors acknowledge financial support from the National Key Technology Research and Development Program of China (2015BAA04B02) and the National Natural Science Foundation of China (No. 51506163).
Nomenclature A
parameter reflecting the thickness of liquid slag layer [m]
Aar
ash content [%]
As,i
barrel inside surface area [m2]
a
emissivity [-]
B
fuel consumption rate for cyclone barrel [kg/s]
D
barrel diameter [m]
Fb
furnace wall area [m2]
f
arrangement density of pin studs [-]
G0
ash mass flow rate entering cyclone barrel [kg/s]
Hr
area of effective radiative heating surface [m2]
I g,e
cyclone barrel exit gas enthalpy [J/kg]
56
L
barrel length [m]
Nuc
Nusselt number [-]
pg
saturated working fluid pressure [MPa]
ps
parameter reflecting the dynamic impact of flue gas on slag film [-]
Q
quantity of heat [J/kg]
q5
radiation and convection heat loss [%]
qF
heat release rate per unit barrel cross-sectional area [kW/m2]
qx
averaged heat flux in the multilayer wall [W/m2]
R
heat resistance [m2·K/W]
Rec
Reynolds number [-]
T
temperature [K]
t
temperature [°C]
t0
coal slag temperature of critical viscosity [°C]
Uc
convective heat transfer coefficient [W/(m2·K)]
u
cyclone barrel perimeter wetted by slag [m]
V
barrel volume [m3]
w2
secondary air velocity [m/s]
wu
circumferential velocity of flue gas [m/s]
x
angular factor [-]
x
averaged angular factor of cyclone barrel [-]
Greek letters
excess air ratio [-]
57
v
viscosity property factor of slag [1/°C]
dip angle [°]
qi
shift in the load range for safety [kW/m2]
layer thickness [m]
b
thermal efficiency of a boiler [%]
s
slag capture efficiency [-]
g,e
cyclone barrel exit gas temperature [°C]
thermal conductivity [W/(m·K)]
viscosity of slag at the temperature of t [Pa·s]
0
critical viscosity of slag [Pa·s]
y
kinematic viscosity of flue gas [m2/s]
s
density of slag [kg/m3]
0
Stefan-Boltzmann constant [W/(m2·K4)]
heat retention factor [-]
Subscripts ar
as-received base
c
convection
daf
dry ash-free base
g
working fluid
g,e
cyclone barrel exit gas
i
index for selected object
m
metal pin studs
58
ph
preheated air
r
radiation
rl
refractory lining
s
slag
s,i
inside surface
wi
water-cooled tube inside surface
wo
water-cooled tube outside surface
y
flue gas
zm,l
liquid slag layer
zm,s
solid slag layer
References [1] J.B. Kitto, S.C. Stultz, Steam – Its Generation and Use, 41th ed., Babcock & Wilcox Company, Charlotte, NC, 2005. [2] S. Wu, W. Bai, C. Tang, X. Tan, C.a. Wang, D. Che, A novel boiler design for high-sodium coal in power generation, in: ASME 2015 Power Conference, POWER 2015, collocated with the ASME 2015 9th International Conference on Energy Sustainability, the ASME 2015 13th International Conference on Fuel Cell Science, Engineering and Technology, and the ASME 2015 Nuclear Forum, June 28, 2015 - July 2, 2015, American Society of Mechanical Engineers (ASME), San Diego, CA, United states, 2015, pp. Power Division. [3] H. Farzan, R.J. Newell, G.J. Maringo, J.D. Riggs, A.S. Yagiela, Evaluation of reburning NOx control and coal switching in a cyclone boiler, in: Proceedings of the Engineering Foundation Conference on Coal-Blending and Switching of Low-Sulfur Western Coals, September 26, 1993 October 1, 1993, Publ by ASME, Snowbird, UT, USA, 1994, pp. 323. [4] H. Farzan, G.J. Maringo, C.T. Beard, G.E. Weed, J. Pratapas, NO x control using natural gas reburn on an industrial cyclone boiler, in: Proceedings of the 1997 22nd International Technical Conference on Coal Utilization & amp; Fuel Systems, March 16, 1997 - March 19, 1997, Coal & 59
Slurry Technology Assoc, Clearwater, FL, USA, 1997, pp. 801-814. [5] B. Smith, S. Voss, T. Titus, Overfire air blows away cyclone NO x, Power Engineering (Barrington, Illinois), 103(1) (1999). [6] H. Gadalla, N. Peters, F. Iman, M. Zhanhua, SmartBurn approach for reduced NO x emissions below 0.20 lb/MMBtu at cyclone-fired boilers, in: COAL-GEN 2007 Conference, August 1, 2007 - August 3, 2007, Unavailable, Milwaukee, WI, United states, 2007. [7] W. Bai, H. Li, L. Deng, H. Liu, D. Che, Air-staged combustion characteristics of pulverized coal under high temperature and strong reducing atmosphere conditions, Energy & Fuels, 28(3) (2014) 1820-1828. [8] C.H. Marston, Thermal interactions in the slag tap of a cyclone coal combustor, in: Heat Transfer in Fire and Combustion Systems. Presented at the 23rd National Heat Transfer Conference., ASME, Denver, CO, USA, 1985, pp. 251-258. [9] Z. Zhou, Y. Bo, Y. Zhang, Z. Huang, L. Chen, L. Ge, J. Zhou, K. Cen, Interactions of high-chromia refractory materials with infiltrating coal slag in the oxidizing atmosphere of a cyclone furnace, Ceramics International, 40(3) (2014) 3829-3839. [10] K. Ohtake, Y. Nakatake, Cold model study of flow field and slag rejection efficiency in cyclone slagging combustor, in: Proceedings of the 3rd ASME/JSME Thermal Engineering Joint Conference Part 5 (of 5), March 17, 1991 - March 22, 1991, Publ by ASME, Reno, NV, USA, 1991, pp. 239-244. [11] Y. Nakatake, I. Naruse, K. Ohtake, Cold model experiment of cyclone slagging gasifier with high slag rejection efficiency and flow field control, Transactions of the Japan Society of Mechanical Engineers, Part B, 61(585) (1995) 1928-1934. [12] H. Yuan, H. Qu, H. Ren, F. Wang, Z. Yu, An experimental study of slag deposit in the entrained-flow gasifier, Journal of East China University of Science and Technology, 31(3) (2005) 393-397 (in Chinese). [13] J. Zhang, Q. Liang, J. Wang, J. Xu, H. Liu, X. Gong, Flow characteristics of slag in Shell gasifier slag bath, Chemical Engineering (China), 39(4) (2011) 89-93 (in Chinese). [14] X. Cheng, G. Hou, Q. Liang, J. Xu, H. Liu, Experimental study of slag flow on membrane wall in entrained-flow gasifier, Chemical Engineering (China), 40(3) (2012) 58-62 (in Chinese). [15] M. Seggiani, Modelling and simulation of time varying slag flow in a Prenflo entrained-flow 60
gasifier, Fuel, 77(14) (1998) 1611-1621. [16] S.Z. Yong, M. Gazzino, A. Ghoniem, Modeling the slag layer in solid fuel gasification and combustion – Formulation and sensitivity analysis, Fuel, 92(1) (2012) 162-170. [17] I. Ye, C. Ryu, Numerical modeling of slag flow and heat transfer on the wall of an entrained coal gasifier, Fuel, 150 (2015) 64-74. [18] I. Ye, C. Ryu, J.H. Koo, Influence of critical viscosity and its temperature on the slag behavior on the wall of an entrained coal gasifier, Applied Thermal Engineering, 87 (2015) 175-184. [19] M. Otaka, H. Watanabe, S. Hara, Numerical simulation of molten-slag flow in coal gasifier, in: 2005 ASME Power Conference, April 5, 2005 - April 7, 2005, American Society of Mechanical Engineers, Chicago, IL, United states, 2005, pp. 1471-1476. [20] D. Wang, X. Ling, H. Peng, Theoretical analysis of free-surface film flow on the rotary granulating disk in waste heat recovery process of molten slag, Applied Thermal Engineering, 63(1) (2014) 387-395. [21] B. Zhao, S.P. Vanka, B.G. Thomas, Numerical study of flow and heat transfer in a molten flux layer, International Journal of Heat and Fluid Flow, 26(1) (2005) 105-118. [22] S. Liu, Y. Hao, Numerical study on slag flow in an entrained-flow gasifier, in: ASME International Mechanical Engineering Congress and Exposition, IMECE 2007, November 11, 2007 - November 15, 2007, American Society of Mechanical Engineers, Seattle, WA, United states, 2008, pp. 793-800. [23] C. Wieland, B. Kreutzkam, G. Balan, H. Spliethoff, Evaluation, comparison and validation of deposition criteria for numerical simulation of slagging, Applied Energy, 93 (2012) 184-192. [24] M. Risberg, P. Carlsson, R. Gebart, Numerical modeling of a 500 kW air-blown cyclone gasifier, Applied Thermal Engineering, 90 (2015) 694-702. [25] A.V. Ryzhakov, Role of slag coating in the dynamics of transient processes of a lined furnace chamber, Teploenergetika, (3) (1973) 50-53. [26] X. Li, G. Li, Z. Cao, S. Xu, Research on flow characteristics of slag film in a slag tapping gasifier, Energy & Fuels, 24(9) (2010) 5109-5115. [27] C. Yan, B. Lin, E. Chen, Liquid slag film thickness solution model and heat transfer analysis for liquid slag-removal burner, Boiler Technology, 32(7) (2001) 7-9 (in Chinese). 61
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Figure Captions Fig. 1. Cyclone barrel pin stud and refractory section Fig. 2. Multilayer wall model of heat transfer in the cyclone barrel Fig. 3. Schematic diagram of liquid slag film along the perimeter of the cyclone barrel Fig. 4. Comparison of the cyclone barrel exit gas temperatures obtained through the existing test data (or CFD) and method of the present study Fig. 5. Load characteristics of the cyclone barrel: (a) fuel consumption rate; (b) averaged heat flux; (c) surface temperature distributions of different layers; (d) thicknesses of liquid and solid slag layers; ( D = 2.4 m, L = 9.6 m, rl = 20 mm, = 1.1, tph = 400 °C and qF0 = 20000 kw/m2) Fig. 6. Effect of barrel diameter on the adaptability ( L /D = 4.0, rl = 20 mm, = 1.1, tph = 400 °C and qF0 = 20000 kw/m2) Fig. 7. Effect of thickness of refractory lining on the adaptability ( D = 2.4 m, L = 9.6 m, = 1.1, tph = 400 °C and qF0 = 20000 kw/m2)
Fig. 8. Effect of excess air ratio on the adaptability ( D = 2.4 m, L = 9.6 m, rl = 20 mm, tph = 400 °C and qF0 = 20000 kw/m2) Fig. 9. Effect of temperature of preheated air on the adaptability ( D = 2.4 m, L = 9.6 m, rl = 20 mm, = 1.1 and qF0 = 20000 kw/m2) Fig. 10. Effect of ash content on the adaptability ( D = 2.4 m, L = 9.6 m, rl = 20 mm, = 1.1, tph = 400 °C and qF0 = 20000 kw/m2)
Fig. 11. Effect of coal slag temperature of critical viscosity on the adaptability ( D = 2.4 m, L = 9.6 m, rl = 20 mm, = 1.1, tph = 400 °C and qF0 = 20000 kw/m2)
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Figures:
Fig. 1
Fig. 2
Fig. 3
64
Fig. 4
Fig. 5
65
Fig. 6
Fig. 7
66
Fig. 8
Fig. 9
67
Fig. 10
Fig. 11
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Table Captions Table 1 Coal property analysis of selected fuel (wt %) Table 2 Main design parameters and operating conditions of a cyclone barrel Table 3 Ultimate analyses of four studied bituminous coals (wt %)
Tables: Table 1 Coal property analysis of selected fuel (wt %) Car
Har
Oar
Nar
Sar
Aar
Mar
Vdaf
Qnet,ar
t0
0
v
%
%
%
%
%
%
%
%
kJ/kg
°C
Pa·s
1/°C
58.86
3.36
6.98
0.79
0.63
19.78
9.60
32.31
22440
1360
14
0.015
Table 2 Main design parameters and operating conditions of a cyclone barrel Design Parameters
Operating Conditions
Parameter
Symbol /(Unit)
Value
Parameter
Symbol /(Unit)
Value
Barrel diameter
D /(m)
2.4
Excess air ratio
/(-)
1.1
tph /(°C)
400
w2 /(m/s)
90
pg /(MPa)
10
tg /(°C)
311.0
Temperature of Barrel length
L /(m)
9.6 preheated air Secondary air
Barrel volume
V /(m3)
43.4 velocity
Barrel inside
Saturated working 2
As,i /(m )
81.4
surface area
fluid pressure
Thickness of
Saturated working
rl /(mm)
20
refractory lining
fluid temperature
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Table 3 Ultimate analyses of four studied bituminous coals (wt %) Car
Har
Oar
Nar
Sar
Aar
Mar
Vdaf
Qnet,ar /(kJ/kg)
69.44
3.99
6.76
0.73
1.40
9.68
8.00
30.70
26820
58.86
3.36
6.98
0.79
0.63
19.78
9.60
32.31
22440
50.50
3.50
6.00
1.00
1.00
30.00
8.00
25.50
20010
40.73
3.42
5.21
0.98
0.77
39.69
9.20
25.00
16800
70
Highlights:
An appropriate method was proposed to evaluate the adaptability of cyclone barrel.
A calculation model and a judgment criterion were developed in the method.
There is a load range for the cyclone barrel to meet the judgment criterion.
Effects of various factors on the adaptability were investigated systematically.
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