- Email: [email protected]
Modeling the slag flow and heat transfer with the effect of fluid-solid slag layer interface viscosity in an entrained flow gasifier
Applied Thermal Engineering 122 (2017) 785–793
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Modeling the slag flow and heat transfer with the effect of fluid-solid slag layer interface viscosity in an entrained flow gasifier Binbin Zhang a,b, Zhongjie Shen a,b, Qinfeng Liang a,b,c, Jianliang Xu a,b, Haifeng Liu a,b,⇑ a Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, East China University of Science and Technology, P.O. Box 272, Shanghai 200237, PR China b Shanghai Engineering Research Center of Coal Gasification, East China University of Science and Technology, P.O. Box 272, Shanghai 200237, PR China c State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, PR China
h i g h l i g h t s Modeling the slag layer characteristics with the effect of fluid-solid slag layer interface viscosity. The smoother the viscosity-temperature profile, the higher the effects of the fluid-solid slag interface viscosity. The critical viscosity can be regarded as the fluid-solid interface viscosity for the crystalline slag.
a r t i c l e
i n f o
Article history: Received 23 December 2016 Revised 20 April 2017 Accepted 22 April 2017 Available online 23 April 2017 Keywords: Gasification Slag Viscosity Flow Heat transfer
a b s t r a c t The characteristics of slag flow and heat transfer in a gasifier are significant for controlling the operation conditions. Determination of the fluid-solid slag layer interface is a crucial procedure in the studying of slag flow and heat transfer characteristics. The varied absolute viscosities were used as the fluid-solid slag layer interface viscosity to model the slag layer properties in an entrained flow gasifier. The results showed that with the increase of fluid-solid slag layer interface viscosity, the liquid slag layer thickness increased, while the solid slag layer thickness and the liquid slag velocity decreased. Moreover, the slag layer overall thickness had a slight decrease, while the slag heat flux had a slight increase. In addition, the effects of the fluid-solid slag layer interface viscosity on slag layer characteristics with glassy slag and plastic slag were relatively higher than crystalline slag. The smoother the viscosity-temperature profile, the higher the influences of the fluid-solid slag layers interface viscosity. The critical viscosity could be approximate regarded as the fluid-solid slag layer interface viscosity when the slag type was crystalline slag during the model derivation, and the fluid-solid interface viscosity can be defined as about 100 Pa s for plastic slag and glassy slag. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Coal gasification was a supporting technology for the energy and chemical industry by converting the solid fuel into hydrogen and carbon monoxide-rich syngas [1]. One of the most widespread gasification technologies was entrained flow gasification on account of its high efficient and extensive of coal species [2]. During the gasification, after the coal was gasified and fusion, most of the coal ash deposited on the wall and formed a slag layer which protected metal wall from the high temperature gas corrosion [3].
⇑ Corresponding author at: Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, East China University of Science and Technology, P.O. Box 272, Shanghai 200237, PR China. E-mail address: [email protected] (H. Liu). http://dx.doi.org/10.1016/j.applthermaleng.2017.04.095 1359-4311/Ó 2017 Elsevier Ltd. All rights reserved.
The slag layer consisted of two phases: the liquid layer heated by the high temperature syngas and the solid layer cooled by the refractory wall [4]. Determination of the interface of solid phase and fluid phase was a crucial factor in the studying of slag layer characteristics. Therefore, the interface criterion of solid and liquid slag phase was a significant research in gasifier. Experiment study for slag flow and heat transfer in an industrial gasifier is limited and difficult due to the high temperature and pressure environment [5–8]. Several models have been proposed to describe the slag flow and heat transfer behavior in entrained flow gasifier. Seggiani [9] had built a simplified model to simulate the time varying slag flow characteristic in a Prenflo entrained flow gasifier, which was widely used in many studies [10–16]. This model was based on the conservation equation and some assumptions. One of most crucial assumption was that the
786
B. Zhang et al. / Applied Thermal Engineering 122 (2017) 785–793
Nomenclature cs D g h mex min qflux qin qout To Tcv Tf Tg Tmr Tmw Tw u x
slag specific heat (J/kg K) gasifier diameter (m) gravitational constant (m/s2) convective heat transfer coefficient (W/m2 K) vertical outlet slag mass flow rate per unit (kg/s) mass flow of particle deposition per unit (kg/s) heat flux through the slag (W/m2) heat flux to the slag surface (W/m2) heat flux from the slag to the refractory layer (W/m2) slag temperature at slag-gas interface (K) temperature of critical viscosity (K) slag flow temperature (K) gas temperature (K) metal-refractory wall interface temperature (K) water-metal wall interface temperature (K) refractory-slag interface temperature (K) slag velocity at distance x from slag-gas interface (m/s) the distance from the slag-gas interface (m)
Greek letters thickness of liquid slag layer (m) dl dm thickness of metal wall (m) thickness of refractory wall (m) dr ds thickness of solid slag layer (m) e emissivity h slope of the wall (°)
transition temperature of liquid and solid slag layers was the temperature of critical viscosity (Tcv), which would greatly simplify the derivation process during the simulation. Yong et al. [17–19] proposed a modified model to simulate the slag layer in solid fuel gasification and combustion, in which the temperature profile was assumed as a cubic polynomial and the considered the shear stress by depositing particles. The slag flow under the temperature between the Tcv and the flow temperature (Tf) was considered for the throat part of gasifier [20]. Li et al. [21] calculated the heat transfer of slag layer by the discretization of the velocity in a black liquor recovery boiler. The heat transfer from numerical analysis and the experimental results was compared in Ref. [22]. Ye et al. [23,24] built the slag flow model with the control volume method, the unique feature was that the deposition from the gasifier was regarded as a new control volume which attached to the original slag. In general, the slag model for a gasifier included slag flow and heat transfer model, particle model and 3-D model [25]. The particle model was mainly to describe the particle deposition or particle capture process [26–29]. The 3-D model was determined by the slag properties and operation conditions of gasifier, which can obtain through the simulation software and formula [30–37]. The slag flow and heat transfer model were based on the conservation equations and some simplified assumptions [38–40], and different assumptions or derivation process led to the various modeling results. In most of the literatures, the temperature of critical viscosity (Tcv) was used as the transition temperature between the liquid and solid slag layers [9,17–19,23,24]. Generally, Tcv referred to the temperature at which the viscosity rapid changes, which could be calculated by the composition of slag [41]. The Tcv is a fixed characteristic parameter for a slag which can be calculated from the formula or obtained from the viscosity-temperature profile. While most of the literature uses the Tcv as the liquid-solid slag interface temperature, without considering how much the impact of this assumption on the calculation results. In this study, we model the slag layer properties with different interface viscosity,
thermal conductivity of liquid slag (W/m K) thermal conductivity of metal wall (W/m K) thermal conductivity of refractory wall (W/m K) thermal conductivity of solid slag (W/m K) liquid slag viscosity (Pa s) solid–liquid slag layer interface viscosity (Pa s) slag density (kg/m3) blackbody radiation coefficient (W/m2 K4)
kl km kr ks
ll lsl q r
Subscripts 0 slag surface or x = 0 critical viscosity cv f slag flow g gas i slag unit in inflow to the slag l liquid slag m matel wall mr matel-refractory wall interface mw metal wall-water interface out outflow from the slag r refractory wall s solid slag sl solid-liquid slag layer interface w refractory wall surface
thus attempting to redefine the liquid-solid slag interface according to the model results. The theoretical knowledge of determination of liquid-solid slag interface is not enough and the purpose of this work is to improve the theoretical knowledge construction. This paper uses the absolute viscosity as interface criterion of solid and liquid slag layers. The objective is to study the effects of the fluid-solid interface viscosity on slag layer properties in entrained flow gasifier. The modified model is based on Seggiani’s [9] work and analyzed using numerical method. Several typical slags (crystalline slag, plastic slag, and glassy slag) have been used to investigate the effects of the fluid-solid interface viscosity with different viscosity-temperature characteristics. 2. Numerical models 2.1. Slag flow and heat transfer model Fig. 1 shows a typical slag layer unit along the wall in an entrained flow gasifier. The slag flow and heat transfer model is established in accordance with Seggiani’s [9] method which based on a series of assumptions. One of the most important assumptions
mex,i-1 Cooling water
Tg Metal wall
Refractory wall
qflux
qflux T mw
T mr
T0
T sl
δs
δl x
Tw Solid layer
Liquid layer
min,i qin Coal ash particle Syngas
mex,i Fig. 1. Schematic diagram of slag layer for unit i along the wall.
787
B. Zhang et al. / Applied Thermal Engineering 122 (2017) 785–793
min;i þ mex;i1 mex;i ¼ 0
ð1Þ
where the min is the mass flow rate of particle deposition in unit i, which is assumed as the average value from the CFD simulation results in gasifier. The mex,i is the leaving slag mass flow rate and the mex,i1 is the incoming slag mass flow in unit i. While the incoming slag mass flow rate of the first unit is 0, the mass flow rate of particle deposition in each unit is known, therefore the leaving slag mass flow rate of the unit i can be calculated by superposition method. The momentum conservation in linear coordinates for cell i can be expressed as:
xi ¼ 0;
i ll;i du ¼0 dxi
xi ¼ dl;i ; ui ¼ 0
T i ¼ T sl T 0;i xi =dl;i þ T 0;i
ð3Þ
where the Ti is the temperature at distance x from the slag surface, T0,i is the temperature at the slag-gas interface, dl is the liquid slag
30 20
0 1600
1640
1800
Fig. 3. Viscosity-temperature characteristics of the blending coal slag.
250
Crystalline slag Plastic slag Glassy slag A Glassy slag B
200
150
100
50
0 1450
1500
1550
1600
1650
1700
1750
1800
Fig. 4. Four typical slag viscosity-temperature characteristics.
4670
layer thickness. Tsl is the temperature at the interface of liquid and solid slag layers, which is obtained from the slag viscositytemperature profile when defining the absolute viscosity at fluidsolid interface. Combining Eq. (4) and slag viscosity-temperature relationship as shown in Figs. 3 and 4, the relationship of the slag viscosity and the distance can be represented using numerical method. Bringing the relationship of the slag viscosity and the distance into Eq. (2), the slag velocity can be derived after integral for Eq. (2). This is a process of solving boundary value problems of ordinary differential equations. A calculation program was written in Matlab software to get the numerical solution of velocity distribution for Eq. (2). After obtaining the velocity distribution of liquid slag, the leaving slag mass flow rate of unit i can be calculated as:
Z
335
144
Burner
Slag
1760
Temperature(K)
8 15mm
Burner
1720
Temperature(K)
2960 Syngas
1680
547
Refractory wall
40
10
ð2Þ
where the ll,i is the liquid slag viscosity in unit i, which is effected by the slag temperature as shown in Figs. 3 and 4, ui is the slag velocity at distance x from slag-gas interface, dl,i is the liquid slag layer thickness, q is the slag density, and h is the slope of the wall. According to the assumption that the temperature profile in liquid slag was linear distribution, the temperature at distance x from the slag surface can be expressed as:
Metal wall
50
Viscosity Pa.s
d dui ðl Þ ¼ qg cos hi ; dxi l;i dxi
60
Viscosity(Pa.s)
in previous study is that the transition temperature between the solid and liquid slag layers is the temperature of critical viscosity (Tcv). However, we consider that the slag is still flow at the viscosity above the critical viscosity, and we assume the liquid-solid slag layer interface viscosity is the absolute value above critical viscosity. Then the mass, momentum, and energy conservation equations for cell i in the steady-state condition can be expressed as follows. The mass conservation for cell i can be expressed as:
Fig. 2. Configuration and the dimensions of the shell gasifier.
mex;i ¼ qpDi
dl;i
ui dxi
ð4Þ
0
where Di is the gasifier diameter. In order to solve Eqs. (2)–(4), a value of liquid slag thickness should be presupposed. An initial value of liquid slag thickness was presupposed to obtain the leaving slag mass flow rate (mex,i) by Eqs. (2)–(4), then compared with the result of the leaving slag mass flow rate (mex,i) from Eq. (1). If the error between the two approach results was within in a small range, thus the initial presupposed value of liquid slag thickness was
788
B. Zhang et al. / Applied Thermal Engineering 122 (2017) 785–793
reasonable. Otherwise, revised the value of liquid slag thickness until the error between the two approach results was within in a small range. A calculation program was written in Matlab software to solve the iterative process to obtain the approximate solution of liquid slag thickness. The liquid slag thickness was obtained from the above iterative process. The energy conservation for cell i in molten slag can be expressed as:
Table 1 Model parameters and boundary conditions. Model parameters and conditions
Value 2
Slag deposition mass flow rate (kg/m s) Gas temperature Tg (K) Cooling water temperature (K) Refractory thermal conductivity kr (W/m K) Metal wall thermal conductivity km (W/m K) Viscosity at the solid-liquid interface lsl (Pa s)
1 1 T o;i1 þ T sl mex;i1 C s ðT o;i þ T sl Þmex;i C s qout;i 2 2 ¼0 ð5Þ
0.0315 1865 523 8 43 6.79 (Tcv), 25, 50, 100, 200, 500
min;i C s T g þ þ qin;i
The radiative heat transfer to the slag surface mainly from the gas, thus the heat flux to the liquid slag can be calculated as:
qin;i ¼ h T g;i T o;i þ er T 4g;i T 4o;i
ð6Þ
The heat flux of slag for cell i can be calculated as follow:
qout;i ¼ qflux;i ¼
T o;i T sl T sl T w;i T w;i T mr;i T mr;i T mw;i ¼ ¼ ¼ dl;i =ks ds;i =ks dr =kr dm =km ð7Þ
The approximate solution of liquid slag thickness was obtained from the above iterative process. Then, the result of liquid slag thickness was brought into Eq. (7) and combined with Eqs. (5) and (6), the other slag flow and heat transfer characteristics were obtained by solving the equations.
temperature profile (the temperature in the point of mutational viscosity). In addition, four typical slags (crystalline slag, plastic slag, glassy slag A, and glassy slag B) have been used in this work, to study the effects of the fluid-solid interface viscosity on slag flow and heat transfer with different slag types. To ensure the unity of the variable, it is necessary to ensure that other parameters remain unchanged. Therefore, we assume that the four slag types have the same Tcv and the viscosity-temperature curves are the same at the temperature above the Tcv, as shown in Fig. 4. The viscosity of crystalline slag rapidly increases when the temperature below Tcv, while the viscosity of glassy slag increases slowly. The viscosity changes of plastic slag is between the crystalline slag and glassy slag when the temperature below Tcv. 3. Results and discussion
2.2. Simulation conditions and slag properties The slag flow and transfer model is tested in a Shell pulverized coal gasifier (2000t/d coal consumption) with boundary conditions from CFD simulation. Fig. 2 shows the configuration and the dimensions of the gasifier. The slag model is applied to straight segment part and the gasifier is evenly subdivided into 11 units along the axial direction. The CFD simulation for Shell gasifier is established according to the Xu’s [14] method. The simulation grid is consistent with industrial plants and the simulation region is set to a quarter of gasifier in order to reduce the computational load. The eddy dissipative concept model is adopted to simulate the homogeneous reactions including gaseous combustion, a watergas shift and methane-steam reaction. The heterogeneous reaction including char gasification is modeled with random pore model. The realizable k–e model is adopted to model the turbulence of gas phase. The Lagrangian coordinates and random trajectory models are used to describe the particle motion. Detailed information for the CFD simulation method can be found in Ref. [14]. After obtaining the CFD simulation results, we adopt the average value of the particles deposition and gas temperature distribution as the initial condition for slag flow model. Table 1 presents the model parameter and the boundary conditions for a gasifier. The average temperature of the cooling water in the membrane wall keeps for about 250 °C, and the metal temperature at the metal-water interface is approximately equal to the average temperature of the water due to the turbulent convection heat transfer. The varied absolute viscosities are used as interface criterion of solid and liquid slag layers in this work. A blending coal slag is used to model the slag flow and heat transfer in this work. The slag viscosity is measured by a hightemperature rotational viscometer for the RV DVIII system (Theta Industries, Port Wahington, NY) and the viscosity-temperature profile is present in Fig. 3. The blending coal slag properties are shown in Table 2, including the temperature of critical viscosity, density, specific heat, thermal conductivity and emissivity. The temperature of critical viscosity is obtained from the viscosity-
3.1. Effects of the fluid-solid interface viscosity on slag flow and heat transfer 3.1.1. Effects of the fluid-solid interface viscosity on liquid slag thickness Fig. 5 shows the liquid slag thickness along the gasifier in different fluid-solid slag interface viscosity. The liquid slag thickness increases along the gasifier wall due to the deposition of slag. The liquid slag thickness increases with the increase of interface viscosity, especially when the interface viscosity changes near the critical viscosity. In terms of the physical model, with the increases of the interface viscosity, the original slag which should be considered as solid phase turn into liquid phase. Therefore, the liquid slag thickness increases with the increase of interface viscosity. From the point of model assumptions and derivation process, the liquid slag thickness is mainly affected by the slag deposition quantity and the liquid slag viscosity. The liquid slag viscosity changed with the increases of interface viscosity, thus the liquid slag thickness also changed. 3.1.2. Effects of the fluid-solid interface viscosity on solid slag thickness Fig. 6 shows the solid slag thickness along the gasifier in different fluid-solid slag interface viscosity. Part of the high temperature molten slag cool into solid slag after transfer the heat to the wall. Therefore, the solid slag thickness increases along the gasifier wall.
Table 2 Properties of the blending coal slag. Slag properties
Value
Temperature of critical viscosity Tcv (K) Critical viscosity lcv (Pa s) Density qs (kg/m3) Specific heat cs (J/kg k) Thermal conductivity ks (W/m K) Emissivity
1618 6.79 2830 1118 1.42 0.83
789
Liquid slag thickness (mm)
2.8 2.6 2.4 2.2 =6.79Pa.s =25Pa.s =50Pa.s =100Pa.s =200Pa.s
2.0 1.8 1.6 1.4 1.2 1.0
0
2
4
6
8
10
12
Liquid slag average thickness (mm)
B. Zhang et al. / Applied Thermal Engineering 122 (2017) 785–793
2.02 2.01 2.00 1.99 1.98 1.97 1.96
Unit i
0
100
200
300
400
500
Fluid-solid slag interface viscosity (Pa.s)
Fig. 5. Effects of the fluid-solid interface viscosity on liquid slag thickness.
5.76
8 7 6 =6.79Pa.s =25Pa.s =50Pa.s =100Pa.s =200Pa.s
5 4 3 2 1
0
2
4
6
8
10
12
Solid slag average thickness (mm)
Solid slag thickness (mm)
9
5.74 5.72 5.70 5.68 5.66 5.64
Unit i
0
100
200
300
400
500
Fluid-solid slag interface viscosity (Pa.s)
Fig. 6. Effects of the fluid-solid interface viscosity on solid slag thickness.
7.72 Slag layer average thickness (mm)
Slag layer thickness (mm)
12 10 8
=6.79Pa.s =25Pa.s =50Pa.s =100Pa.s =200Pa.s
6 4 2
0
2
4
6
8
10
12
Unit i
7.71 7.70 7.69 7.68 7.67 7.66 7.65
0
100
200
300
400
500
Fluid-solid slag interface viscosity (Pa.s)
Fig. 7. Effects of the fluid-solid interface viscosity on slag layer overall thickness.
The solid slag thickness decreases with the increase of interface viscosity as shown in Fig. 6, especially when the fluid-solid interface viscosity changes near the critical viscosity. In terms of the physical model, with the increases of the fluid-solid interface viscosity, the original slag which should be considered as solid phase turn into liquid phase. Therefore, the solid slag thickness decreases with the increases of interface viscosity. From the point of model derivation process, the liquid slag thickness has slight increase and the interface temperature decreases with the increase of fluid-solid interface viscosity, so the heat flux of slag has been changed with the increases of interface viscosity. Therefore, the
liquid slag thickness also changed with the increases of interface viscosity according to Eq. (5). 3.1.3. Effects of the fluid-solid interface viscosity on slag overall thickness Fig. 7 shows the slag overall thickness along the gasifier in the different fluid-solid slag interface viscosity. The slag overall thickness increases along the gasifier wall due to the molten slag deposition and solidification. The slag overall thickness has a slight decrease with the increases of interface viscosity. In terms of the physical model, the change of fluid-solid interface viscosity affects
B. Zhang et al. / Applied Thermal Engineering 122 (2017) 785–793
3.5
Liquid slag average velocity (cm/s)
Liquid slag average velocity (cm/s)
790
3.0 2.5 2.0
=6.79Pa.s =25Pa.s =50Pa.s =100Pa.s =200Pa.s
1.5 1.0 0.5 0.0
0
2
4
6
8
10
12
1.97 1.96 1.95 1.94 1.93 1.92 1.91 1.90
Unit i
0
100
200
300
400
500
Fluid-solid slag interface viscosity (Pa.s)
Fig. 8. Effects of the fluid-solid interface viscosity on liquid slag average velocity.
193.0 Slag average heat flux ( kw/m 2 )
Slag heat flux (kw/m 2 )
350 =6.79Pa.s =25Pa.s =50Pa.s =100Pa.s =200Pa.s
300 250 200 150 100
0
2
4
6
8
10
12
Unit i
192.5
192.0
191.5
191.0
0
100
200
300
400
500
Fluid-solid slag interface viscosity (Pa.s)
Fig. 9. Effects of the fluid-solid interface viscosity on heat flux of slag.
the division of the liquid-solid slag interface, and the effect on slag overall thickness is uncertain. According to the previous results, the increase of interface viscosity increases the liquid slag thickness and decreases the solid slag thickness. In view of the solid slag layer is thicker than the liquid slag layer, thus the change of solid slag plays a leader role on the change of slag overall thickness.
the point of model derivation process, the liquid slag thickness increases and the interface temperature decreases with the increase of interface viscosity, thus the heat flux of slag changes with the effect of interface viscosity according Eq. (5).
3.1.4. Effects of the fluid-solid interface viscosity on liquid slag average velocity Fig. 8 shows the liquid slag average velocity along the gasifier in the different fluid-solid interface viscosity. The liquid slag velocity increases along the gasifier wall due to the gravitational potential energy. In addition, the liquid slag average velocity has a slight decrease with the increase of interface viscosity. In terms of the physical model, the increase of interface viscosity increases the liquid slag viscosity, thus the increased viscosity will hinder the fluid flow. From the point of model derivation process, the liquid slag velocity is mainly affected by the slag deposition quantity and the liquid slag viscosity.
Four typical slag (crystalline slag, plastic slag, glassy slag A, and glassy slag B) has been used to model the slag flow and heat transfer. Four types of slag have different results due to the large difference of slag viscosity-temperature characteristics. The slag flow and heat transfer feature shows different changes under the change of fluid-solid slag interface viscosity with the four types of slag.
3.1.5. Effects of the fluid-solid interface viscosity on heat flux of slag Fig. 9 shows the heat flux of slag along the gasifier in the different fluid-solid slag interface viscosity. The heat flux of slag decreases along the gasifier due to the increase of the slag layer thickness. The heat flux of slag has a slight increase with the increase of interface viscosity. According to the previous results, the slag layer thickness has a slight decrease with the increase of interface viscosity, thus the heat flux of slag has a slight increase when the range of slag temperature almost remain the same. From
3.2. Results with different slag viscosity-temperature profile
3.2.1. Liquid slag average thickness with different slag types Fig. 10 shows the effects of the fluid-solid slag interface viscosity on liquid slag average thickness with different slag types. The liquid slag average thickness increases with the increase of interface viscosity, especially when the interface viscosity changes near the critical viscosity. The liquid layer thickness increasing range of glassy slag is bigger than the plastic slag, and the liquid layer thickness increasing range of plastic slag is bigger than the crystalline slag. Combining Figs. 4 and 10, the liquid slag layer thickness increasing range is bigger when the viscosity-temperature curve is smoother. The fluid-solid interface has different temperature with the increase of interface viscosity for different slag types. With the increase of interface viscosity, the interface temperature of crystalline slag almost remains the same, while the interface
791
Liquid slag average thickness (mm)
B. Zhang et al. / Applied Thermal Engineering 122 (2017) 785–793
3.2
thinner than the crystalline slag with the increase of interface viscosity. Similarly, the solid layer thickness decreasing range of plastic slag with the increase of interface viscosity is between the crystalline slag and glassy slag. The blending slag in above section can be seen as crystalline slag, thus the modeling results for solid slag thickness is similar to crystalline slag as shown in Fig 11.
Crystalline slag Plastic slag Glassy slag A Glassy slag B
3.0 2.8 2.6 2.4 2.2 2.0 1.8
0
50
100
150
200
Fluid-solid slag interface viscosity (Pa.s) Fig. 10. Effects of the fluid-solid interface viscosity on liquid slag average thickness with different slag types.
temperature of glassy slag B drops a lot. The decrease of the interface temperature has a significant impact on liquid slag thickness. From the point of slag characteristics, the crystalline slag solidification more easily than the glassy slag under the same condition. Therefore, the liquid slag thickness of glassy slag is thicker than the crystalline slag with the increase of interface viscosity. Similarly, the liquid layer thickness increasing range of plastic slag with the increase of interface viscosity is between the crystalline slag and glassy slag. The blending slag in above section can be seen as crystalline slag, thus the modeling results for liquid slag thickness is similar to crystalline slag as shown in Fig 10. 3.2.2. Solid slag average thickness with different slag types Fig. 11 shows the effects of the fluid-solid slag interface viscosity on solid slag average thickness with different slag types. The liquid slag average thickness decreases with the increase of interface viscosity, especially when the interface viscosity changes near the critical viscosity. The solid layer thickness decreasing range of glassy slag is bigger than the plastic slag, and the solid layer thickness decreasing range of plastic slag is bigger than the crystalline slag. Combining Figs. 4 and 11, the solid slag layer thickness decreasing range is bigger when the viscosity-temperature curve is smoother. From the point of slag characteristics, the crystalline slag solidification more easily than the glassy slag under the same condition. Therefore, the solid slag thickness of glassy slag is
3.2.3. The slag layer average thickness with different slag types Fig. 12 shows the effects of the fluid-solid slag interface viscosity on slag layer overall thickness with different slag types. The slag layer overall thickness has a slight decrease with the increase of interface viscosity, especially when the interface viscosity changes near the critical viscosity. Similarly, the slag layer overall thickness decreasing range of glassy slag is bigger than the plastic slag, and the plastic slag is bigger than the crystalline slag. As described in previous discussion, the solid slag layer is thicker than the liquid slag layer, thus the change of solid slag plays a leader role on the change of slag layer overall thickness. Therefore, the slag layer overall thickness of glassy slag is thinner than the crystalline slag with the increase of interface viscosity, and the plastic slag is between the crystalline slag and glassy slag. However, the reduction rate of slag layer overall thickness of glassy slag B is only about 6% when the fluid-solid interface viscosity is 200 Pa s. The modeling result of slag overall thickness for blending slag in above section is similar to crystalline slag as shown in Fig 12. 3.2.4. Liquid slag average velocity with different slag types Fig. 13 shows the effects of the fluid-solid slag interface viscosity on liquid slag average velocity with different slag types. The liquid slag average velocity decreases with the increase of interface viscosity, especially when the interface viscosity changes near the critical viscosity. With the increase of fluid-solid interface viscosity, the liquid slag average velocity of crystalline slag almost remains the same, while the liquid slag average velocity of glassy slag B drops a lot. According to the previous results, the liquid slag thickness of glassy slag is thicker than the crystalline slag with the increase of interface viscosity. Under the condition of the slag deposition quantity are the same, which means the mass flow rate of liquid slag are the same, the fluid flow velocity is slower when the liquid slag layer is thicker. Therefore, the liquid slag velocity of glassy slag is slower than the crystalline slag with the increase of interface viscosity, and the plastic slag is between the crystalline slag and glassy slag. In addition, the modeling result of slag velocity for blending slag in above section is similar to crystalline slag as shown in Fig 13.
8.0
5.8
Slag layer average thickness (mm)
Solid slag average thickness (mm)
6.0
5.6 5.4 5.2 Crystalline slag Plastic slag Glassy slag A Glassy slag B
5.0 4.8 4.6 4.4 4.2 4.0
7.8
7.6
7.4
7.2
7.0
0
50
100
150
200
Fluid-solid slag interface viscosity (Pa.s) Fig. 11. Effects of the fluid-solid interface viscosity on solid slag average thickness with different slag types.
Crystalline slag Plastic slag Glassy slag A Glassy slag B
0
50
100
150
200
Fluid-solid slag interface viscosity (Pa.s) Fig. 12. Effects of the fluid-solid interface viscosity on slag layer overall thickness with different slag types.
792
B. Zhang et al. / Applied Thermal Engineering 122 (2017) 785–793
have different effects on modeling results. In addition, there is a significant conclusion from the model result: For the crystalline slag, the model results almost remain the same with the increased interface viscosity, thus the critical viscosity can be considered as the fluid-solid interface viscosity. But for the other slag types, this method will produce a larger error when calculating the slag flow characteristics. The flow characteristics for plastic slag and glassy slag change with the fluid-solid interface viscosity mainly before 100 Pa s. Therefore, the fluid-solid interface viscosity can be defined as about 100 Pa s for plastic slag and glassy slag.
Liquid slag average velocity (cm/s)
2.0
1.8
Crystalline slag Plastic slag Glassy slag A Glassy slag B
1.6
1.4
4. Conclusions
1.2
0
50
100
150
200
Fluid-solid slag interface viscosity (Pa.s) Fig. 13. Effects of the fluid-solid interface viscosity on liquid slag average velocity with different slag types.
The slag average heat flux (kw/m2)
202 200
Crystalline slag Plastic slag Glassy slag A Glassy slag B
198 196 194 192 190
0
50
100
150
200
Fluid-solid slag interface viscosity (Pa.s) Fig. 14. Effects of the fluid-solid interface viscosity on the average heat flux of slag with different slag types.
The effects of the fluid-solid slag layers interface viscosity on slag flow and heat transfer characteristics were study for an entrained flow gasifier. With the application of one blending coal slag and four typical slags, the main conclusions are as follows: (1) With the increase of fluid-solid interface viscosity, the liquid slag layer thickness increases, while the solid slag layer thickness and the liquid slag velocity decreases. Moreover, the slag layer overall thickness has a slight decrease, while the slag heat flux has a slight increase. (2) The slag flow and heat transfer characteristics mainly change at the viscosity near the critical viscosity, and the characteristic values of slag flow and heat transfer will be convergence with the increase of fluid-solid interface viscosity. (3) The effects of the fluid-solid slag layers interface viscosity on slag flow and heat transfer characteristics with glassy slag and plastic slag are relatively higher than crystalline slag. The smoother the viscosity-temperature profile, the higher the effects of the fluid-solid slag layers interface viscosity. (4) The critical viscosity can be approximated regarded as the fluid-solid interface viscosity when the slag type in gasifier is crystalline slag, which can simplify the model calculation. Moreover, the fluid-solid interface viscosity can be defined as about 100 Pa s for plastic slag and glassy slag.
Acknowledgement 3.2.5. The average slag heat flux of different slag types Fig. 14 shows the effects of the fluid-solid slag interface viscosity on slag heat flux with different slag types. The slag average heat flux increases with the increase of interface viscosity, especially when the fluid-solid interface viscosity changes near the critical viscosity. With the increase of interface viscosity, the heat flux of crystalline slag almost remains the same, while the heat flux of glassy slag B has s slight increase. The growth rate of glassy slag B heat flux is only about 5% when the interface viscosity is 200 Pa s. According to the previous results, the slag layer overall thickness only has a slight decrease with the increase of interface viscosity, therefore, the heat flux of slag only has a slight increase when the range of slag temperature almost remain the same. Therefore, the interface viscosity has a slight effect on heat flux of slag. In addition, the modeling result of slag heat flux for blending slag in above section is similar to crystalline slag as shown in Fig 14. From the above modeling results we can find that the interface viscosities of different slag types have different effects on slag flow and heat transfer. Different slag types have different viscositytemperature characteristics, thus the interface temperature will be different when the interface is the same. It means the inputs parameters such as Tsl are different in equations for the model calculation. Therefore the interface viscosities of different slag types
This study was supported by the Foundation of Shanghai Science and Technology Committee (14dz1200100), the National Natural Science Foundation of China (U1402272), the National Natural Science Foundation of China (21376082), and the Foundation of State Key Laboratory of Coal Conversion (Grant No. J16-17-301). References [1] X. Gong, W. Lu, X. Guo, Z. Dai, Q. Liang, H. Liu, H. Zhang, B. Guo, Pilot-scale comparison investigation of different entrained-flow gasification technologies and prediction on industrial-scale gasification performance, Fuel 129 (2014) 37–44. [2] L. Kong, J. Bai, W. Li, X. Wen, X. Li, Z. Bai, Z. Guo, H. Li, The internal and external factor on coal ash slag viscosity at high temperatures, Part 3: Effect of CaO on the pattern of viscosity–temperature curves of slag, Fuel 179 (2016) 10–16. [3] W. Xuan, J. Zhang, D. Xia, Crystallization characteristics of a coal slag and influence of crystals on the sharp increase of viscosity, Fuel 176 (2016) 102– 109. [4] W. Lin, Q. Liang, G. Yu, H. Liu, X. Gong, Numerical modeling for non-steady thermal stress analysis of slag layer in a membrane wall entrained-flow gasifier, Fuel 90 (2011) 2396–2403. [5] S. Hossein, R. Gupta, Inorganic matter behavior during coal gasification: effect of operating conditions and particle trajectory on ash deposition and slag formation, Energy Fuels 29 (2015) 1503–1519. [6] J. Wang, H. Liu, Q. Liang, J. Xu, Experimental and numerical study on slag deposition and growth at the slag tap hole region of shell gasifier, Fuel Process. Technol. 106 (2013) 704–711.
B. Zhang et al. / Applied Thermal Engineering 122 (2017) 785–793 [7] Q. Liang, X. Guo, Z. Dai, H. Liu, X. Gong, An investigation on the heat transfer behavior and slag deposition of membrane wall in pilot-scale entrained-flow gasifier, Fuel 102 (2012) 491–498. [8] J. Ni, G. Yu, Q. Guo, Q. Liang, Z. Zhou, Experimental and numerical study of the flow field and temperature field for a large-scale radiant syngas cooler, Ind. Eng. Chem. Res. 49 (2010) 4452–4461. [9] M. Seggiani, Modelling and simulation of time varying slag flow in a Prenflo entrained-flow gasifier, Fuel 77 (1998) 1611–1621. [10] M.J. Bockelie, M.K. Denison, Z. Chen, T. Linjewile, C.L. Senior, A.F. Sarofim, CFD modeling for entrained flow gasifiers in vision 21 systems, in: Proceedings of the 19th Annual International Pittsburgh Coal Conference, 2002. [11] X. Wang, D. Zhao, L. Jiang, W. Yang, The deposition and burning characteristics during slagging co-firing coal and wood: modeling and numerical simulation, Combust Sci. Technol. 181 (2009) 710–728. [12] X. Wang, D. Zhao, L. He, L. Jiang, Q. He, Y. Chen, Modeling of a coal-fired slagging combustor: development of a slag submodel, Combust. Flame 149 (2007) 249–260. [13] S. Liu, Y. Hao, Numerical study on slag flow in an entrained-flow gasifier, in: ASME International Mechanical Engineering Congress and Exposition, Washington, USA, 2007. [14] J. Xu, Q. Liang, Z. Dai, H. Liu, Comprehensive model with time limited wall reaction for entrained flow gasifier, Fuel 184 (2016) 118–127. [15] B. Zhang, Z. She, D. Han, Q. Liang, J. Xu, H. Liu, Effects of the bubbles in slag on slag flow and heat transfer in the membrane wall entrained-flow gasifier, Appl. Therm. Eng. 112 (2017) 1178–1186. [16] B. Sun, Y. Liu, X. Chen, Q. Zhou, M. Su, Dynamic modeling and simulation of shell gasifier in IGCC, Fuel Process. Technol. 92 (2011) 1418–1425. [17] S.Z. Yong, M. Gazzino, A.F. Ghoniem, Modeling the slag layer in solid fuel gasification and combustion-formulation and sensitivity analysis, Fuel 92 (2012) 162–170. [18] S.Z. Yong, A.F. Ghoniem, Modeling the slag layer in solid fuel gasification and combustion-Two-way coupling with CFD, Fuel 97 (2012) 457–466. [19] L. Chen, S.Z. Yong, A.F. Ghoniem, Modeling the slag behavior in three dimensional CFD simulation of a vertically-oriented oxy-coal combustor, Fuel Process. Technol. 112 (2013) 106–117. [20] J. Ni, Z. Zhou, G. Yu, Q. Liang, F. Wang, Molten slag flow and phase transformation behaviors in a slagging entrained-flow coal gasifier, Ind. Eng. Chem. Res. 49 (2010) 12302–12310. [21] B. Li, A. Brink, M. Hupa, Simplified model for determining local heat flux boundary conditions for slagging wall, Energy Fuels 23 (2009) 3418–3422. [22] J. Kittel, F. Hannemann, F. Mehlhose, S. Heil, B. Meyer, Dynamic modeling of the heat transfer into the cooling screen of the SFGT-gasifier, in: 7th Modelica Conference, Como, Italy, 2009. [23] 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, Appl. Therm. Eng. 87 (2015) 175–184. [24] 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.
793
[25] D. Bi, Q. Guan, W. Xuan, J. Zhang, Combined slag flow model for entrained flow gasification, Fuel 150 (2015) 565–572. [26] P. Pednekar, D. Bhattacharyya, J.S. Kasule, R. Turton, R. Rengaswamy, Dynamic model of a slagging entrained-flow gasifier including models of slag transport, deposition, and slag layer, Ind. Eng. Chem. Res. 55 (2016) 279–292. [27] W. Ai, J.M. Kuhlman, Simulation of coal ash particle deposition experiments, Energy Fuels 25 (2011) 708–718. [28] A.A. Bhuiyan, J. Naser, Modelling of slag deposition and flow characteristics of coal combustion under oxy-firing condition in a 550 MW tangentially fired furnace, Appl. Therm. Eng. 106 (2016) 221–235. [29] J. Ni, G. Yu, Q. Guo, Z. Zhou, F. Wang, Submodel for predicting slag deposition formation in slagging gasification systems, Energy Fuels 25 (2011) 1004–1009. [30] G. Yu, J. Ni, Q. Liang, Q. Guo, Z. Zhou, Modeling of multiphase flow and heat transfer in radiant syngas cooler of an entrained-flow coal gasification, Ind. Eng. Chem. Res. 48 (2009) 10094–10103. [31] A. Slezak, J.M. Kuhlman, L.J. Shadle, J. Spenik, S. Shi, CFD simulation of entrained-flow coal gasification: coal particle density/size fraction effects, Powder Technol. 203 (2010) 98–108. [32] H.J. Jeong, D.K. Seo, J. Hwang, CFD modeling for coal size effect on coal gasification in a two-stagecommercial entrained-bed gasifier with an improved char gasification model, Appl. Energy 103 (2014) 29–36. [33] J. Xu, H. Zhao, Z. Dai, H. Liu, F. Wang, Numerical simulation of opposed multiburner gasifier under different coal loading ratio, Fuel 174 (2016) 97–106. [34] C. Chen, M. Horio, T. Kojima, Use of numerical modeling in the design and scale-up of entrained flow coal gasifier, Fuel 80 (2001) 1513–1523. [35] Y. Wu, J. Zhang, P. Smith, H. Zhang, R. Charles, J. Lv, G. Yue, Three-dimensional simulation for an entrained flow coal slurry gasifier, Energy Fuels 24 (2010) 1156–1163. [36] M. Kumar, A.F. Ghoniem, Multiphysics simulations of entrained flow gasification. Part I: Validating the non-reacting flow solver and the particle turbulent dispersion model, Energy Fuels 26 (2012) 451–463. [37] J. Ni, G. Yu, Q. Guo, Z. Dai, F. Wang, Modeling and comparison of different syngas cooling types for entrained-flow gasifier, Chem. Eng. Sci. 66 (2011) 448–459. [38] R.F.D. Monaghan, A.F. Ghoniem, A dynamic reduced order model for simulating entrained flow gasifiers, Part II: Model validation and sensitivity analysis, Fuel 94 (2012) 280–297. [39] H.H. Lee, J.C. Lee, Y.J. Joo, M. Oh, C.H. Lee, Dynamic modeling of shell entrained flow gasifier in an integrated gasification combined cycle process, Appl. Energy 131 (2014) 425–440. [40] Z. Yang, Z. Wang, Y. Wu, J. Wang, J. Lu, Z. Li, W. Ni, Dynamic model for an oxygen-staged slagging entrained flow gasifier, Energy Fuels 25 (2011) 3646– 3656. [41] S. Vargas, F.J. Frandsen, K. Dam-Johansen, Rheological properties of high temperature melts of coal ashes and other silicates, Prog. Energy Combust. Sci. 27 (2001) 237–429.
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Modeling the slag flow and heat transfer with the effect of fluid-solid slag layer interface viscosity in an entrained flow gasifier Binbin Zhang a,b, Zhongjie Shen a,b, Qinfeng Liang a,b,c, Jianliang Xu a,b, Haifeng Liu a,b,⇑ a Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, East China University of Science and Technology, P.O. Box 272, Shanghai 200237, PR China b Shanghai Engineering Research Center of Coal Gasification, East China University of Science and Technology, P.O. Box 272, Shanghai 200237, PR China c State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, PR China
h i g h l i g h t s Modeling the slag layer characteristics with the effect of fluid-solid slag layer interface viscosity. The smoother the viscosity-temperature profile, the higher the effects of the fluid-solid slag interface viscosity. The critical viscosity can be regarded as the fluid-solid interface viscosity for the crystalline slag.
a r t i c l e
i n f o
Article history: Received 23 December 2016 Revised 20 April 2017 Accepted 22 April 2017 Available online 23 April 2017 Keywords: Gasification Slag Viscosity Flow Heat transfer
a b s t r a c t The characteristics of slag flow and heat transfer in a gasifier are significant for controlling the operation conditions. Determination of the fluid-solid slag layer interface is a crucial procedure in the studying of slag flow and heat transfer characteristics. The varied absolute viscosities were used as the fluid-solid slag layer interface viscosity to model the slag layer properties in an entrained flow gasifier. The results showed that with the increase of fluid-solid slag layer interface viscosity, the liquid slag layer thickness increased, while the solid slag layer thickness and the liquid slag velocity decreased. Moreover, the slag layer overall thickness had a slight decrease, while the slag heat flux had a slight increase. In addition, the effects of the fluid-solid slag layer interface viscosity on slag layer characteristics with glassy slag and plastic slag were relatively higher than crystalline slag. The smoother the viscosity-temperature profile, the higher the influences of the fluid-solid slag layers interface viscosity. The critical viscosity could be approximate regarded as the fluid-solid slag layer interface viscosity when the slag type was crystalline slag during the model derivation, and the fluid-solid interface viscosity can be defined as about 100 Pa s for plastic slag and glassy slag. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Coal gasification was a supporting technology for the energy and chemical industry by converting the solid fuel into hydrogen and carbon monoxide-rich syngas [1]. One of the most widespread gasification technologies was entrained flow gasification on account of its high efficient and extensive of coal species [2]. During the gasification, after the coal was gasified and fusion, most of the coal ash deposited on the wall and formed a slag layer which protected metal wall from the high temperature gas corrosion [3].
⇑ Corresponding author at: Key Laboratory of Coal Gasification and Energy Chemical Engineering of Ministry of Education, East China University of Science and Technology, P.O. Box 272, Shanghai 200237, PR China. E-mail address: [email protected] (H. Liu). http://dx.doi.org/10.1016/j.applthermaleng.2017.04.095 1359-4311/Ó 2017 Elsevier Ltd. All rights reserved.
The slag layer consisted of two phases: the liquid layer heated by the high temperature syngas and the solid layer cooled by the refractory wall [4]. Determination of the interface of solid phase and fluid phase was a crucial factor in the studying of slag layer characteristics. Therefore, the interface criterion of solid and liquid slag phase was a significant research in gasifier. Experiment study for slag flow and heat transfer in an industrial gasifier is limited and difficult due to the high temperature and pressure environment [5–8]. Several models have been proposed to describe the slag flow and heat transfer behavior in entrained flow gasifier. Seggiani [9] had built a simplified model to simulate the time varying slag flow characteristic in a Prenflo entrained flow gasifier, which was widely used in many studies [10–16]. This model was based on the conservation equation and some assumptions. One of most crucial assumption was that the
786
B. Zhang et al. / Applied Thermal Engineering 122 (2017) 785–793
Nomenclature cs D g h mex min qflux qin qout To Tcv Tf Tg Tmr Tmw Tw u x
slag specific heat (J/kg K) gasifier diameter (m) gravitational constant (m/s2) convective heat transfer coefficient (W/m2 K) vertical outlet slag mass flow rate per unit (kg/s) mass flow of particle deposition per unit (kg/s) heat flux through the slag (W/m2) heat flux to the slag surface (W/m2) heat flux from the slag to the refractory layer (W/m2) slag temperature at slag-gas interface (K) temperature of critical viscosity (K) slag flow temperature (K) gas temperature (K) metal-refractory wall interface temperature (K) water-metal wall interface temperature (K) refractory-slag interface temperature (K) slag velocity at distance x from slag-gas interface (m/s) the distance from the slag-gas interface (m)
Greek letters thickness of liquid slag layer (m) dl dm thickness of metal wall (m) thickness of refractory wall (m) dr ds thickness of solid slag layer (m) e emissivity h slope of the wall (°)
transition temperature of liquid and solid slag layers was the temperature of critical viscosity (Tcv), which would greatly simplify the derivation process during the simulation. Yong et al. [17–19] proposed a modified model to simulate the slag layer in solid fuel gasification and combustion, in which the temperature profile was assumed as a cubic polynomial and the considered the shear stress by depositing particles. The slag flow under the temperature between the Tcv and the flow temperature (Tf) was considered for the throat part of gasifier [20]. Li et al. [21] calculated the heat transfer of slag layer by the discretization of the velocity in a black liquor recovery boiler. The heat transfer from numerical analysis and the experimental results was compared in Ref. [22]. Ye et al. [23,24] built the slag flow model with the control volume method, the unique feature was that the deposition from the gasifier was regarded as a new control volume which attached to the original slag. In general, the slag model for a gasifier included slag flow and heat transfer model, particle model and 3-D model [25]. The particle model was mainly to describe the particle deposition or particle capture process [26–29]. The 3-D model was determined by the slag properties and operation conditions of gasifier, which can obtain through the simulation software and formula [30–37]. The slag flow and heat transfer model were based on the conservation equations and some simplified assumptions [38–40], and different assumptions or derivation process led to the various modeling results. In most of the literatures, the temperature of critical viscosity (Tcv) was used as the transition temperature between the liquid and solid slag layers [9,17–19,23,24]. Generally, Tcv referred to the temperature at which the viscosity rapid changes, which could be calculated by the composition of slag [41]. The Tcv is a fixed characteristic parameter for a slag which can be calculated from the formula or obtained from the viscosity-temperature profile. While most of the literature uses the Tcv as the liquid-solid slag interface temperature, without considering how much the impact of this assumption on the calculation results. In this study, we model the slag layer properties with different interface viscosity,
thermal conductivity of liquid slag (W/m K) thermal conductivity of metal wall (W/m K) thermal conductivity of refractory wall (W/m K) thermal conductivity of solid slag (W/m K) liquid slag viscosity (Pa s) solid–liquid slag layer interface viscosity (Pa s) slag density (kg/m3) blackbody radiation coefficient (W/m2 K4)
kl km kr ks
ll lsl q r
Subscripts 0 slag surface or x = 0 critical viscosity cv f slag flow g gas i slag unit in inflow to the slag l liquid slag m matel wall mr matel-refractory wall interface mw metal wall-water interface out outflow from the slag r refractory wall s solid slag sl solid-liquid slag layer interface w refractory wall surface
thus attempting to redefine the liquid-solid slag interface according to the model results. The theoretical knowledge of determination of liquid-solid slag interface is not enough and the purpose of this work is to improve the theoretical knowledge construction. This paper uses the absolute viscosity as interface criterion of solid and liquid slag layers. The objective is to study the effects of the fluid-solid interface viscosity on slag layer properties in entrained flow gasifier. The modified model is based on Seggiani’s [9] work and analyzed using numerical method. Several typical slags (crystalline slag, plastic slag, and glassy slag) have been used to investigate the effects of the fluid-solid interface viscosity with different viscosity-temperature characteristics. 2. Numerical models 2.1. Slag flow and heat transfer model Fig. 1 shows a typical slag layer unit along the wall in an entrained flow gasifier. The slag flow and heat transfer model is established in accordance with Seggiani’s [9] method which based on a series of assumptions. One of the most important assumptions
mex,i-1 Cooling water
Tg Metal wall
Refractory wall
qflux
qflux T mw
T mr
T0
T sl
δs
δl x
Tw Solid layer
Liquid layer
min,i qin Coal ash particle Syngas
mex,i Fig. 1. Schematic diagram of slag layer for unit i along the wall.
787
B. Zhang et al. / Applied Thermal Engineering 122 (2017) 785–793
min;i þ mex;i1 mex;i ¼ 0
ð1Þ
where the min is the mass flow rate of particle deposition in unit i, which is assumed as the average value from the CFD simulation results in gasifier. The mex,i is the leaving slag mass flow rate and the mex,i1 is the incoming slag mass flow in unit i. While the incoming slag mass flow rate of the first unit is 0, the mass flow rate of particle deposition in each unit is known, therefore the leaving slag mass flow rate of the unit i can be calculated by superposition method. The momentum conservation in linear coordinates for cell i can be expressed as:
xi ¼ 0;
i ll;i du ¼0 dxi
xi ¼ dl;i ; ui ¼ 0
T i ¼ T sl T 0;i xi =dl;i þ T 0;i
ð3Þ
where the Ti is the temperature at distance x from the slag surface, T0,i is the temperature at the slag-gas interface, dl is the liquid slag
30 20
0 1600
1640
1800
Fig. 3. Viscosity-temperature characteristics of the blending coal slag.
250
Crystalline slag Plastic slag Glassy slag A Glassy slag B
200
150
100
50
0 1450
1500
1550
1600
1650
1700
1750
1800
Fig. 4. Four typical slag viscosity-temperature characteristics.
4670
layer thickness. Tsl is the temperature at the interface of liquid and solid slag layers, which is obtained from the slag viscositytemperature profile when defining the absolute viscosity at fluidsolid interface. Combining Eq. (4) and slag viscosity-temperature relationship as shown in Figs. 3 and 4, the relationship of the slag viscosity and the distance can be represented using numerical method. Bringing the relationship of the slag viscosity and the distance into Eq. (2), the slag velocity can be derived after integral for Eq. (2). This is a process of solving boundary value problems of ordinary differential equations. A calculation program was written in Matlab software to get the numerical solution of velocity distribution for Eq. (2). After obtaining the velocity distribution of liquid slag, the leaving slag mass flow rate of unit i can be calculated as:
Z
335
144
Burner
Slag
1760
Temperature(K)
8 15mm
Burner
1720
Temperature(K)
2960 Syngas
1680
547
Refractory wall
40
10
ð2Þ
where the ll,i is the liquid slag viscosity in unit i, which is effected by the slag temperature as shown in Figs. 3 and 4, ui is the slag velocity at distance x from slag-gas interface, dl,i is the liquid slag layer thickness, q is the slag density, and h is the slope of the wall. According to the assumption that the temperature profile in liquid slag was linear distribution, the temperature at distance x from the slag surface can be expressed as:
Metal wall
50
Viscosity Pa.s
d dui ðl Þ ¼ qg cos hi ; dxi l;i dxi
60
Viscosity(Pa.s)
in previous study is that the transition temperature between the solid and liquid slag layers is the temperature of critical viscosity (Tcv). However, we consider that the slag is still flow at the viscosity above the critical viscosity, and we assume the liquid-solid slag layer interface viscosity is the absolute value above critical viscosity. Then the mass, momentum, and energy conservation equations for cell i in the steady-state condition can be expressed as follows. The mass conservation for cell i can be expressed as:
Fig. 2. Configuration and the dimensions of the shell gasifier.
mex;i ¼ qpDi
dl;i
ui dxi
ð4Þ
0
where Di is the gasifier diameter. In order to solve Eqs. (2)–(4), a value of liquid slag thickness should be presupposed. An initial value of liquid slag thickness was presupposed to obtain the leaving slag mass flow rate (mex,i) by Eqs. (2)–(4), then compared with the result of the leaving slag mass flow rate (mex,i) from Eq. (1). If the error between the two approach results was within in a small range, thus the initial presupposed value of liquid slag thickness was
788
B. Zhang et al. / Applied Thermal Engineering 122 (2017) 785–793
reasonable. Otherwise, revised the value of liquid slag thickness until the error between the two approach results was within in a small range. A calculation program was written in Matlab software to solve the iterative process to obtain the approximate solution of liquid slag thickness. The liquid slag thickness was obtained from the above iterative process. The energy conservation for cell i in molten slag can be expressed as:
Table 1 Model parameters and boundary conditions. Model parameters and conditions
Value 2
Slag deposition mass flow rate (kg/m s) Gas temperature Tg (K) Cooling water temperature (K) Refractory thermal conductivity kr (W/m K) Metal wall thermal conductivity km (W/m K) Viscosity at the solid-liquid interface lsl (Pa s)
1 1 T o;i1 þ T sl mex;i1 C s ðT o;i þ T sl Þmex;i C s qout;i 2 2 ¼0 ð5Þ
0.0315 1865 523 8 43 6.79 (Tcv), 25, 50, 100, 200, 500
min;i C s T g þ þ qin;i
The radiative heat transfer to the slag surface mainly from the gas, thus the heat flux to the liquid slag can be calculated as:
qin;i ¼ h T g;i T o;i þ er T 4g;i T 4o;i
ð6Þ
The heat flux of slag for cell i can be calculated as follow:
qout;i ¼ qflux;i ¼
T o;i T sl T sl T w;i T w;i T mr;i T mr;i T mw;i ¼ ¼ ¼ dl;i =ks ds;i =ks dr =kr dm =km ð7Þ
The approximate solution of liquid slag thickness was obtained from the above iterative process. Then, the result of liquid slag thickness was brought into Eq. (7) and combined with Eqs. (5) and (6), the other slag flow and heat transfer characteristics were obtained by solving the equations.
temperature profile (the temperature in the point of mutational viscosity). In addition, four typical slags (crystalline slag, plastic slag, glassy slag A, and glassy slag B) have been used in this work, to study the effects of the fluid-solid interface viscosity on slag flow and heat transfer with different slag types. To ensure the unity of the variable, it is necessary to ensure that other parameters remain unchanged. Therefore, we assume that the four slag types have the same Tcv and the viscosity-temperature curves are the same at the temperature above the Tcv, as shown in Fig. 4. The viscosity of crystalline slag rapidly increases when the temperature below Tcv, while the viscosity of glassy slag increases slowly. The viscosity changes of plastic slag is between the crystalline slag and glassy slag when the temperature below Tcv. 3. Results and discussion
2.2. Simulation conditions and slag properties The slag flow and transfer model is tested in a Shell pulverized coal gasifier (2000t/d coal consumption) with boundary conditions from CFD simulation. Fig. 2 shows the configuration and the dimensions of the gasifier. The slag model is applied to straight segment part and the gasifier is evenly subdivided into 11 units along the axial direction. The CFD simulation for Shell gasifier is established according to the Xu’s [14] method. The simulation grid is consistent with industrial plants and the simulation region is set to a quarter of gasifier in order to reduce the computational load. The eddy dissipative concept model is adopted to simulate the homogeneous reactions including gaseous combustion, a watergas shift and methane-steam reaction. The heterogeneous reaction including char gasification is modeled with random pore model. The realizable k–e model is adopted to model the turbulence of gas phase. The Lagrangian coordinates and random trajectory models are used to describe the particle motion. Detailed information for the CFD simulation method can be found in Ref. [14]. After obtaining the CFD simulation results, we adopt the average value of the particles deposition and gas temperature distribution as the initial condition for slag flow model. Table 1 presents the model parameter and the boundary conditions for a gasifier. The average temperature of the cooling water in the membrane wall keeps for about 250 °C, and the metal temperature at the metal-water interface is approximately equal to the average temperature of the water due to the turbulent convection heat transfer. The varied absolute viscosities are used as interface criterion of solid and liquid slag layers in this work. A blending coal slag is used to model the slag flow and heat transfer in this work. The slag viscosity is measured by a hightemperature rotational viscometer for the RV DVIII system (Theta Industries, Port Wahington, NY) and the viscosity-temperature profile is present in Fig. 3. The blending coal slag properties are shown in Table 2, including the temperature of critical viscosity, density, specific heat, thermal conductivity and emissivity. The temperature of critical viscosity is obtained from the viscosity-
3.1. Effects of the fluid-solid interface viscosity on slag flow and heat transfer 3.1.1. Effects of the fluid-solid interface viscosity on liquid slag thickness Fig. 5 shows the liquid slag thickness along the gasifier in different fluid-solid slag interface viscosity. The liquid slag thickness increases along the gasifier wall due to the deposition of slag. The liquid slag thickness increases with the increase of interface viscosity, especially when the interface viscosity changes near the critical viscosity. In terms of the physical model, with the increases of the interface viscosity, the original slag which should be considered as solid phase turn into liquid phase. Therefore, the liquid slag thickness increases with the increase of interface viscosity. From the point of model assumptions and derivation process, the liquid slag thickness is mainly affected by the slag deposition quantity and the liquid slag viscosity. The liquid slag viscosity changed with the increases of interface viscosity, thus the liquid slag thickness also changed. 3.1.2. Effects of the fluid-solid interface viscosity on solid slag thickness Fig. 6 shows the solid slag thickness along the gasifier in different fluid-solid slag interface viscosity. Part of the high temperature molten slag cool into solid slag after transfer the heat to the wall. Therefore, the solid slag thickness increases along the gasifier wall.
Table 2 Properties of the blending coal slag. Slag properties
Value
Temperature of critical viscosity Tcv (K) Critical viscosity lcv (Pa s) Density qs (kg/m3) Specific heat cs (J/kg k) Thermal conductivity ks (W/m K) Emissivity
1618 6.79 2830 1118 1.42 0.83
789
Liquid slag thickness (mm)
2.8 2.6 2.4 2.2 =6.79Pa.s =25Pa.s =50Pa.s =100Pa.s =200Pa.s
2.0 1.8 1.6 1.4 1.2 1.0
0
2
4
6
8
10
12
Liquid slag average thickness (mm)
B. Zhang et al. / Applied Thermal Engineering 122 (2017) 785–793
2.02 2.01 2.00 1.99 1.98 1.97 1.96
Unit i
0
100
200
300
400
500
Fluid-solid slag interface viscosity (Pa.s)
Fig. 5. Effects of the fluid-solid interface viscosity on liquid slag thickness.
5.76
8 7 6 =6.79Pa.s =25Pa.s =50Pa.s =100Pa.s =200Pa.s
5 4 3 2 1
0
2
4
6
8
10
12
Solid slag average thickness (mm)
Solid slag thickness (mm)
9
5.74 5.72 5.70 5.68 5.66 5.64
Unit i
0
100
200
300
400
500
Fluid-solid slag interface viscosity (Pa.s)
Fig. 6. Effects of the fluid-solid interface viscosity on solid slag thickness.
7.72 Slag layer average thickness (mm)
Slag layer thickness (mm)
12 10 8
=6.79Pa.s =25Pa.s =50Pa.s =100Pa.s =200Pa.s
6 4 2
0
2
4
6
8
10
12
Unit i
7.71 7.70 7.69 7.68 7.67 7.66 7.65
0
100
200
300
400
500
Fluid-solid slag interface viscosity (Pa.s)
Fig. 7. Effects of the fluid-solid interface viscosity on slag layer overall thickness.
The solid slag thickness decreases with the increase of interface viscosity as shown in Fig. 6, especially when the fluid-solid interface viscosity changes near the critical viscosity. In terms of the physical model, with the increases of the fluid-solid interface viscosity, the original slag which should be considered as solid phase turn into liquid phase. Therefore, the solid slag thickness decreases with the increases of interface viscosity. From the point of model derivation process, the liquid slag thickness has slight increase and the interface temperature decreases with the increase of fluid-solid interface viscosity, so the heat flux of slag has been changed with the increases of interface viscosity. Therefore, the
liquid slag thickness also changed with the increases of interface viscosity according to Eq. (5). 3.1.3. Effects of the fluid-solid interface viscosity on slag overall thickness Fig. 7 shows the slag overall thickness along the gasifier in the different fluid-solid slag interface viscosity. The slag overall thickness increases along the gasifier wall due to the molten slag deposition and solidification. The slag overall thickness has a slight decrease with the increases of interface viscosity. In terms of the physical model, the change of fluid-solid interface viscosity affects
B. Zhang et al. / Applied Thermal Engineering 122 (2017) 785–793
3.5
Liquid slag average velocity (cm/s)
Liquid slag average velocity (cm/s)
790
3.0 2.5 2.0
=6.79Pa.s =25Pa.s =50Pa.s =100Pa.s =200Pa.s
1.5 1.0 0.5 0.0
0
2
4
6
8
10
12
1.97 1.96 1.95 1.94 1.93 1.92 1.91 1.90
Unit i
0
100
200
300
400
500
Fluid-solid slag interface viscosity (Pa.s)
Fig. 8. Effects of the fluid-solid interface viscosity on liquid slag average velocity.
193.0 Slag average heat flux ( kw/m 2 )
Slag heat flux (kw/m 2 )
350 =6.79Pa.s =25Pa.s =50Pa.s =100Pa.s =200Pa.s
300 250 200 150 100
0
2
4
6
8
10
12
Unit i
192.5
192.0
191.5
191.0
0
100
200
300
400
500
Fluid-solid slag interface viscosity (Pa.s)
Fig. 9. Effects of the fluid-solid interface viscosity on heat flux of slag.
the division of the liquid-solid slag interface, and the effect on slag overall thickness is uncertain. According to the previous results, the increase of interface viscosity increases the liquid slag thickness and decreases the solid slag thickness. In view of the solid slag layer is thicker than the liquid slag layer, thus the change of solid slag plays a leader role on the change of slag overall thickness.
the point of model derivation process, the liquid slag thickness increases and the interface temperature decreases with the increase of interface viscosity, thus the heat flux of slag changes with the effect of interface viscosity according Eq. (5).
3.1.4. Effects of the fluid-solid interface viscosity on liquid slag average velocity Fig. 8 shows the liquid slag average velocity along the gasifier in the different fluid-solid interface viscosity. The liquid slag velocity increases along the gasifier wall due to the gravitational potential energy. In addition, the liquid slag average velocity has a slight decrease with the increase of interface viscosity. In terms of the physical model, the increase of interface viscosity increases the liquid slag viscosity, thus the increased viscosity will hinder the fluid flow. From the point of model derivation process, the liquid slag velocity is mainly affected by the slag deposition quantity and the liquid slag viscosity.
Four typical slag (crystalline slag, plastic slag, glassy slag A, and glassy slag B) has been used to model the slag flow and heat transfer. Four types of slag have different results due to the large difference of slag viscosity-temperature characteristics. The slag flow and heat transfer feature shows different changes under the change of fluid-solid slag interface viscosity with the four types of slag.
3.1.5. Effects of the fluid-solid interface viscosity on heat flux of slag Fig. 9 shows the heat flux of slag along the gasifier in the different fluid-solid slag interface viscosity. The heat flux of slag decreases along the gasifier due to the increase of the slag layer thickness. The heat flux of slag has a slight increase with the increase of interface viscosity. According to the previous results, the slag layer thickness has a slight decrease with the increase of interface viscosity, thus the heat flux of slag has a slight increase when the range of slag temperature almost remain the same. From
3.2. Results with different slag viscosity-temperature profile
3.2.1. Liquid slag average thickness with different slag types Fig. 10 shows the effects of the fluid-solid slag interface viscosity on liquid slag average thickness with different slag types. The liquid slag average thickness increases with the increase of interface viscosity, especially when the interface viscosity changes near the critical viscosity. The liquid layer thickness increasing range of glassy slag is bigger than the plastic slag, and the liquid layer thickness increasing range of plastic slag is bigger than the crystalline slag. Combining Figs. 4 and 10, the liquid slag layer thickness increasing range is bigger when the viscosity-temperature curve is smoother. The fluid-solid interface has different temperature with the increase of interface viscosity for different slag types. With the increase of interface viscosity, the interface temperature of crystalline slag almost remains the same, while the interface
791
Liquid slag average thickness (mm)
B. Zhang et al. / Applied Thermal Engineering 122 (2017) 785–793
3.2
thinner than the crystalline slag with the increase of interface viscosity. Similarly, the solid layer thickness decreasing range of plastic slag with the increase of interface viscosity is between the crystalline slag and glassy slag. The blending slag in above section can be seen as crystalline slag, thus the modeling results for solid slag thickness is similar to crystalline slag as shown in Fig 11.
Crystalline slag Plastic slag Glassy slag A Glassy slag B
3.0 2.8 2.6 2.4 2.2 2.0 1.8
0
50
100
150
200
Fluid-solid slag interface viscosity (Pa.s) Fig. 10. Effects of the fluid-solid interface viscosity on liquid slag average thickness with different slag types.
temperature of glassy slag B drops a lot. The decrease of the interface temperature has a significant impact on liquid slag thickness. From the point of slag characteristics, the crystalline slag solidification more easily than the glassy slag under the same condition. Therefore, the liquid slag thickness of glassy slag is thicker than the crystalline slag with the increase of interface viscosity. Similarly, the liquid layer thickness increasing range of plastic slag with the increase of interface viscosity is between the crystalline slag and glassy slag. The blending slag in above section can be seen as crystalline slag, thus the modeling results for liquid slag thickness is similar to crystalline slag as shown in Fig 10. 3.2.2. Solid slag average thickness with different slag types Fig. 11 shows the effects of the fluid-solid slag interface viscosity on solid slag average thickness with different slag types. The liquid slag average thickness decreases with the increase of interface viscosity, especially when the interface viscosity changes near the critical viscosity. The solid layer thickness decreasing range of glassy slag is bigger than the plastic slag, and the solid layer thickness decreasing range of plastic slag is bigger than the crystalline slag. Combining Figs. 4 and 11, the solid slag layer thickness decreasing range is bigger when the viscosity-temperature curve is smoother. From the point of slag characteristics, the crystalline slag solidification more easily than the glassy slag under the same condition. Therefore, the solid slag thickness of glassy slag is
3.2.3. The slag layer average thickness with different slag types Fig. 12 shows the effects of the fluid-solid slag interface viscosity on slag layer overall thickness with different slag types. The slag layer overall thickness has a slight decrease with the increase of interface viscosity, especially when the interface viscosity changes near the critical viscosity. Similarly, the slag layer overall thickness decreasing range of glassy slag is bigger than the plastic slag, and the plastic slag is bigger than the crystalline slag. As described in previous discussion, the solid slag layer is thicker than the liquid slag layer, thus the change of solid slag plays a leader role on the change of slag layer overall thickness. Therefore, the slag layer overall thickness of glassy slag is thinner than the crystalline slag with the increase of interface viscosity, and the plastic slag is between the crystalline slag and glassy slag. However, the reduction rate of slag layer overall thickness of glassy slag B is only about 6% when the fluid-solid interface viscosity is 200 Pa s. The modeling result of slag overall thickness for blending slag in above section is similar to crystalline slag as shown in Fig 12. 3.2.4. Liquid slag average velocity with different slag types Fig. 13 shows the effects of the fluid-solid slag interface viscosity on liquid slag average velocity with different slag types. The liquid slag average velocity decreases with the increase of interface viscosity, especially when the interface viscosity changes near the critical viscosity. With the increase of fluid-solid interface viscosity, the liquid slag average velocity of crystalline slag almost remains the same, while the liquid slag average velocity of glassy slag B drops a lot. According to the previous results, the liquid slag thickness of glassy slag is thicker than the crystalline slag with the increase of interface viscosity. Under the condition of the slag deposition quantity are the same, which means the mass flow rate of liquid slag are the same, the fluid flow velocity is slower when the liquid slag layer is thicker. Therefore, the liquid slag velocity of glassy slag is slower than the crystalline slag with the increase of interface viscosity, and the plastic slag is between the crystalline slag and glassy slag. In addition, the modeling result of slag velocity for blending slag in above section is similar to crystalline slag as shown in Fig 13.
8.0
5.8
Slag layer average thickness (mm)
Solid slag average thickness (mm)
6.0
5.6 5.4 5.2 Crystalline slag Plastic slag Glassy slag A Glassy slag B
5.0 4.8 4.6 4.4 4.2 4.0
7.8
7.6
7.4
7.2
7.0
0
50
100
150
200
Fluid-solid slag interface viscosity (Pa.s) Fig. 11. Effects of the fluid-solid interface viscosity on solid slag average thickness with different slag types.
Crystalline slag Plastic slag Glassy slag A Glassy slag B
0
50
100
150
200
Fluid-solid slag interface viscosity (Pa.s) Fig. 12. Effects of the fluid-solid interface viscosity on slag layer overall thickness with different slag types.
792
B. Zhang et al. / Applied Thermal Engineering 122 (2017) 785–793
have different effects on modeling results. In addition, there is a significant conclusion from the model result: For the crystalline slag, the model results almost remain the same with the increased interface viscosity, thus the critical viscosity can be considered as the fluid-solid interface viscosity. But for the other slag types, this method will produce a larger error when calculating the slag flow characteristics. The flow characteristics for plastic slag and glassy slag change with the fluid-solid interface viscosity mainly before 100 Pa s. Therefore, the fluid-solid interface viscosity can be defined as about 100 Pa s for plastic slag and glassy slag.
Liquid slag average velocity (cm/s)
2.0
1.8
Crystalline slag Plastic slag Glassy slag A Glassy slag B
1.6
1.4
4. Conclusions
1.2
0
50
100
150
200
Fluid-solid slag interface viscosity (Pa.s) Fig. 13. Effects of the fluid-solid interface viscosity on liquid slag average velocity with different slag types.
The slag average heat flux (kw/m2)
202 200
Crystalline slag Plastic slag Glassy slag A Glassy slag B
198 196 194 192 190
0
50
100
150
200
Fluid-solid slag interface viscosity (Pa.s) Fig. 14. Effects of the fluid-solid interface viscosity on the average heat flux of slag with different slag types.
The effects of the fluid-solid slag layers interface viscosity on slag flow and heat transfer characteristics were study for an entrained flow gasifier. With the application of one blending coal slag and four typical slags, the main conclusions are as follows: (1) With the increase of fluid-solid interface viscosity, the liquid slag layer thickness increases, while the solid slag layer thickness and the liquid slag velocity decreases. Moreover, the slag layer overall thickness has a slight decrease, while the slag heat flux has a slight increase. (2) The slag flow and heat transfer characteristics mainly change at the viscosity near the critical viscosity, and the characteristic values of slag flow and heat transfer will be convergence with the increase of fluid-solid interface viscosity. (3) The effects of the fluid-solid slag layers interface viscosity on slag flow and heat transfer characteristics with glassy slag and plastic slag are relatively higher than crystalline slag. The smoother the viscosity-temperature profile, the higher the effects of the fluid-solid slag layers interface viscosity. (4) The critical viscosity can be approximated regarded as the fluid-solid interface viscosity when the slag type in gasifier is crystalline slag, which can simplify the model calculation. Moreover, the fluid-solid interface viscosity can be defined as about 100 Pa s for plastic slag and glassy slag.
Acknowledgement 3.2.5. The average slag heat flux of different slag types Fig. 14 shows the effects of the fluid-solid slag interface viscosity on slag heat flux with different slag types. The slag average heat flux increases with the increase of interface viscosity, especially when the fluid-solid interface viscosity changes near the critical viscosity. With the increase of interface viscosity, the heat flux of crystalline slag almost remains the same, while the heat flux of glassy slag B has s slight increase. The growth rate of glassy slag B heat flux is only about 5% when the interface viscosity is 200 Pa s. According to the previous results, the slag layer overall thickness only has a slight decrease with the increase of interface viscosity, therefore, the heat flux of slag only has a slight increase when the range of slag temperature almost remain the same. Therefore, the interface viscosity has a slight effect on heat flux of slag. In addition, the modeling result of slag heat flux for blending slag in above section is similar to crystalline slag as shown in Fig 14. From the above modeling results we can find that the interface viscosities of different slag types have different effects on slag flow and heat transfer. Different slag types have different viscositytemperature characteristics, thus the interface temperature will be different when the interface is the same. It means the inputs parameters such as Tsl are different in equations for the model calculation. Therefore the interface viscosities of different slag types
This study was supported by the Foundation of Shanghai Science and Technology Committee (14dz1200100), the National Natural Science Foundation of China (U1402272), the National Natural Science Foundation of China (21376082), and the Foundation of State Key Laboratory of Coal Conversion (Grant No. J16-17-301). References [1] X. Gong, W. Lu, X. Guo, Z. Dai, Q. Liang, H. Liu, H. Zhang, B. Guo, Pilot-scale comparison investigation of different entrained-flow gasification technologies and prediction on industrial-scale gasification performance, Fuel 129 (2014) 37–44. [2] L. Kong, J. Bai, W. Li, X. Wen, X. Li, Z. Bai, Z. Guo, H. Li, The internal and external factor on coal ash slag viscosity at high temperatures, Part 3: Effect of CaO on the pattern of viscosity–temperature curves of slag, Fuel 179 (2016) 10–16. [3] W. Xuan, J. Zhang, D. Xia, Crystallization characteristics of a coal slag and influence of crystals on the sharp increase of viscosity, Fuel 176 (2016) 102– 109. [4] W. Lin, Q. Liang, G. Yu, H. Liu, X. Gong, Numerical modeling for non-steady thermal stress analysis of slag layer in a membrane wall entrained-flow gasifier, Fuel 90 (2011) 2396–2403. [5] S. Hossein, R. Gupta, Inorganic matter behavior during coal gasification: effect of operating conditions and particle trajectory on ash deposition and slag formation, Energy Fuels 29 (2015) 1503–1519. [6] J. Wang, H. Liu, Q. Liang, J. Xu, Experimental and numerical study on slag deposition and growth at the slag tap hole region of shell gasifier, Fuel Process. Technol. 106 (2013) 704–711.
B. Zhang et al. / Applied Thermal Engineering 122 (2017) 785–793 [7] Q. Liang, X. Guo, Z. Dai, H. Liu, X. Gong, An investigation on the heat transfer behavior and slag deposition of membrane wall in pilot-scale entrained-flow gasifier, Fuel 102 (2012) 491–498. [8] J. Ni, G. Yu, Q. Guo, Q. Liang, Z. Zhou, Experimental and numerical study of the flow field and temperature field for a large-scale radiant syngas cooler, Ind. Eng. Chem. Res. 49 (2010) 4452–4461. [9] M. Seggiani, Modelling and simulation of time varying slag flow in a Prenflo entrained-flow gasifier, Fuel 77 (1998) 1611–1621. [10] M.J. Bockelie, M.K. Denison, Z. Chen, T. Linjewile, C.L. Senior, A.F. Sarofim, CFD modeling for entrained flow gasifiers in vision 21 systems, in: Proceedings of the 19th Annual International Pittsburgh Coal Conference, 2002. [11] X. Wang, D. Zhao, L. Jiang, W. Yang, The deposition and burning characteristics during slagging co-firing coal and wood: modeling and numerical simulation, Combust Sci. Technol. 181 (2009) 710–728. [12] X. Wang, D. Zhao, L. He, L. Jiang, Q. He, Y. Chen, Modeling of a coal-fired slagging combustor: development of a slag submodel, Combust. Flame 149 (2007) 249–260. [13] S. Liu, Y. Hao, Numerical study on slag flow in an entrained-flow gasifier, in: ASME International Mechanical Engineering Congress and Exposition, Washington, USA, 2007. [14] J. Xu, Q. Liang, Z. Dai, H. Liu, Comprehensive model with time limited wall reaction for entrained flow gasifier, Fuel 184 (2016) 118–127. [15] B. Zhang, Z. She, D. Han, Q. Liang, J. Xu, H. Liu, Effects of the bubbles in slag on slag flow and heat transfer in the membrane wall entrained-flow gasifier, Appl. Therm. Eng. 112 (2017) 1178–1186. [16] B. Sun, Y. Liu, X. Chen, Q. Zhou, M. Su, Dynamic modeling and simulation of shell gasifier in IGCC, Fuel Process. Technol. 92 (2011) 1418–1425. [17] S.Z. Yong, M. Gazzino, A.F. Ghoniem, Modeling the slag layer in solid fuel gasification and combustion-formulation and sensitivity analysis, Fuel 92 (2012) 162–170. [18] S.Z. Yong, A.F. Ghoniem, Modeling the slag layer in solid fuel gasification and combustion-Two-way coupling with CFD, Fuel 97 (2012) 457–466. [19] L. Chen, S.Z. Yong, A.F. Ghoniem, Modeling the slag behavior in three dimensional CFD simulation of a vertically-oriented oxy-coal combustor, Fuel Process. Technol. 112 (2013) 106–117. [20] J. Ni, Z. Zhou, G. Yu, Q. Liang, F. Wang, Molten slag flow and phase transformation behaviors in a slagging entrained-flow coal gasifier, Ind. Eng. Chem. Res. 49 (2010) 12302–12310. [21] B. Li, A. Brink, M. Hupa, Simplified model for determining local heat flux boundary conditions for slagging wall, Energy Fuels 23 (2009) 3418–3422. [22] J. Kittel, F. Hannemann, F. Mehlhose, S. Heil, B. Meyer, Dynamic modeling of the heat transfer into the cooling screen of the SFGT-gasifier, in: 7th Modelica Conference, Como, Italy, 2009. [23] 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, Appl. Therm. Eng. 87 (2015) 175–184. [24] 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.
793
[25] D. Bi, Q. Guan, W. Xuan, J. Zhang, Combined slag flow model for entrained flow gasification, Fuel 150 (2015) 565–572. [26] P. Pednekar, D. Bhattacharyya, J.S. Kasule, R. Turton, R. Rengaswamy, Dynamic model of a slagging entrained-flow gasifier including models of slag transport, deposition, and slag layer, Ind. Eng. Chem. Res. 55 (2016) 279–292. [27] W. Ai, J.M. Kuhlman, Simulation of coal ash particle deposition experiments, Energy Fuels 25 (2011) 708–718. [28] A.A. Bhuiyan, J. Naser, Modelling of slag deposition and flow characteristics of coal combustion under oxy-firing condition in a 550 MW tangentially fired furnace, Appl. Therm. Eng. 106 (2016) 221–235. [29] J. Ni, G. Yu, Q. Guo, Z. Zhou, F. Wang, Submodel for predicting slag deposition formation in slagging gasification systems, Energy Fuels 25 (2011) 1004–1009. [30] G. Yu, J. Ni, Q. Liang, Q. Guo, Z. Zhou, Modeling of multiphase flow and heat transfer in radiant syngas cooler of an entrained-flow coal gasification, Ind. Eng. Chem. Res. 48 (2009) 10094–10103. [31] A. Slezak, J.M. Kuhlman, L.J. Shadle, J. Spenik, S. Shi, CFD simulation of entrained-flow coal gasification: coal particle density/size fraction effects, Powder Technol. 203 (2010) 98–108. [32] H.J. Jeong, D.K. Seo, J. Hwang, CFD modeling for coal size effect on coal gasification in a two-stagecommercial entrained-bed gasifier with an improved char gasification model, Appl. Energy 103 (2014) 29–36. [33] J. Xu, H. Zhao, Z. Dai, H. Liu, F. Wang, Numerical simulation of opposed multiburner gasifier under different coal loading ratio, Fuel 174 (2016) 97–106. [34] C. Chen, M. Horio, T. Kojima, Use of numerical modeling in the design and scale-up of entrained flow coal gasifier, Fuel 80 (2001) 1513–1523. [35] Y. Wu, J. Zhang, P. Smith, H. Zhang, R. Charles, J. Lv, G. Yue, Three-dimensional simulation for an entrained flow coal slurry gasifier, Energy Fuels 24 (2010) 1156–1163. [36] M. Kumar, A.F. Ghoniem, Multiphysics simulations of entrained flow gasification. Part I: Validating the non-reacting flow solver and the particle turbulent dispersion model, Energy Fuels 26 (2012) 451–463. [37] J. Ni, G. Yu, Q. Guo, Z. Dai, F. Wang, Modeling and comparison of different syngas cooling types for entrained-flow gasifier, Chem. Eng. Sci. 66 (2011) 448–459. [38] R.F.D. Monaghan, A.F. Ghoniem, A dynamic reduced order model for simulating entrained flow gasifiers, Part II: Model validation and sensitivity analysis, Fuel 94 (2012) 280–297. [39] H.H. Lee, J.C. Lee, Y.J. Joo, M. Oh, C.H. Lee, Dynamic modeling of shell entrained flow gasifier in an integrated gasification combined cycle process, Appl. Energy 131 (2014) 425–440. [40] Z. Yang, Z. Wang, Y. Wu, J. Wang, J. Lu, Z. Li, W. Ni, Dynamic model for an oxygen-staged slagging entrained flow gasifier, Energy Fuels 25 (2011) 3646– 3656. [41] S. Vargas, F.J. Frandsen, K. Dam-Johansen, Rheological properties of high temperature melts of coal ashes and other silicates, Prog. Energy Combust. Sci. 27 (2001) 237–429.