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Ind. Eng. Chem. Res. 2010, 49, 4452–4461
Experimental and Numerical Study of the Flow Field and Temperature Field for a Large-Scale Radiant Syngas Cooler Jianjun Ni, Guangsuo Yu,* Qinghua Guo, Qinfeng Liang, and Zhijie Zhou* Key Laboratory of Coal Gasification of Ministry of Education, East China UniVersity of Science and Technology, Shanghai 200237, China
Experimental and numerical studies on the gas-particle flow field and temperature field in the industrial-scale radiant syngas cooler (RSC) have been carried out. The bench-scale cold model experiment was presented for measuring the gas flow field in the RSC. The accuracy and performance of four turbulent flow models were evaluated according to the comparison of predicted results with experimental results. The ash particle trajectories were predicted by the stochastic Lagrangian model and the interaction between the gas and particle phase was also considered by two-way coupling method. A discrete ordinate model (DOM) was used for solving the radiative heat transfer equation when the radiative properties were calculated by weighted-sumof-gray-gases model (WSGGM). The Ranz-Marshall correlation for the Nusselt number was used to account for convection heat-transfer between the gas phase and the particle phase. The ash particle radiative heat transfer was also considered. The physical properties of gas mixtures were calculated by the mass-weightedmixing law. The results indicate that the inlet jet flow intensity is dependent on the inlet diameter, but the length of the jet is independent of the inlet diameter. The thickness of the deposition on the membrane wall has great influence on the heat-transfer. The average temperature profiles of particle are higher than gas in the inner cylinder, and it is inversed in the annular. Furthermore, the results show that particles sizes smaller than 580 µm will be entrained into the annular, but the particle size between 400 and 580 µm cannot be entrained out. And the escaping particle’s temperature is lower than the critical temperature when the deposition thickness is 0.2 mm. 1. Introduction Entrained-flow gasifiers are prime candidates in advanced integrated coal gasification combined cycle (IGCC) power plants. The entrained-flow gasifiers are the most widely used gasifiers with eight different technologies (Shell, Prenflo, Texaco, MHI, E-Gas, GSP, Eagle, and OMB) available.1 In these gasifiers, coal and other solid fuel particles react concurrently with the steam and oxygen or air in the gasifier fluid flow region. Entrained-flow gasifiers usually run at higher temperatures of 1200-1600 °C and pressures of about 2-8 MPa, but most plants run at around 4.0 and 6.5 MPa. The raw syngas and slag exiting from the gasifier usually require significant cooling before being cleaned. Two main methods can be used for cooling the syngas and slag, one using a radiant syngas cooler (RSC), which can also recover the sensible heat of the syngas and molten slag, another by water quenching directly.2 And the energy utilization efficiency will be increased by 4-5% when the RSC and convection syngas cooler (CSC) are used. The hot syngas (or with molten slag) flows through the transition from the gasifier to the RSC, then the sensible heat of the syngas and molten slag will be recovered by the membrane wall of the RSC. Most of the slag particles will be captured by the slag pool at the bottom of the RSC and removed through a lock hopper. The RSC belongs to the critical components of coal gasification plants. It is necessary to understand the physical and dynamic processes involved in the RSC through multiscale simulation, which will direct us how to optimize RSC design and plant layout and to increase the reliability, availability, and maintainability of gasification-based energy systems on the premise of lower costs. The molten and dry slag particle * To whom correspondence should be addressed. Tel.: +86-2164252974. Fax: +86 21 64251312. E-mail address:
[email protected].
deposition on the membrane wall will create fouling, which is difficult to clean by rapping. The fouling will reduce the rate of heat-transfer and can lead to operation failure as reported by many researchers.3-5 On the other hand, the slag/ash depositions also contribute membrane wall corrosion and erosion problems.6,7 So, the cooling process should be carry out without liquid or sticky slag particles touching the heat-transfer surface and causing fouling. The experiment and modeling study of the flow field should be performed in order to make sure that the flow patterns can achieve this required design. However, most studies focused on the details about slag/ash particle deposition, and the results discussed by the analysis tools, such as computercontroled scanning electron microscope (SEM).8,9 All that can help to explain the mechanism of the deposition, but it is difficult to relate this knowledge to optimizing the design for the unknown multiphase flow field in the RSC. Jenkins10 advised that the flow field investigation should be performed to confirm the original designs and to determine that ash particle impingement on the wall could be minimized. A computational fluid dynamics (CFD) model has been applied to predict the multiphase flow fields of different types of industrial equipment.11-13 Kraft et al.14 developed an RSC and introduced a CFD model for prediction of the flow and the heat-transfer process in the RSC, but the details of the mathematic model and the results were not presented. Wessel et al.30 developed a mathematic model and computer program to predict the flow field and temperature field for boiler design and optimization. But all CFD models have not been validated by experimentally measured data or industrial operation data. The original work about multiphase flow and the heat-transfer process in an industrial-scale octahedral RSC has been studied.15,16 The present work focuses on the gas-particle flow field and temperature field in the industrial-scale cylindraceous RSC. The single gas phase flow field measurement experiments in bench-
10.1021/ie100014r 2010 American Chemical Society Published on Web 04/02/2010
Ind. Eng. Chem. Res., Vol. 49, No. 9, 2010
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Figure 2. Schematic diagram of radiant syngas cooler experimental system: (1) raidant syngas cooler, (2, 12) valve, (3) vortex flow meter, (4) fan, (5) hot-wire anemometry system, (6) computer, (7) gas outlet, (8) water inlet, (9) water outlet, (10) probe, (11) purge valve, (13) thermometer. Table 1. Geometry of the Radiant Syngas Cooler Considered parameter (m) Figure 1. Schematic and grid configuration of radiant syngas cooler considered.
scale cold model are presented, and the comparisons of the results obtained by prediction with measurements are discussed. On the other hand, the influences of the inlet structure and the deposition thickness on the membrane wall are also studied. Finally, the primary purification processes of the ash particles with different diameters are analyzed. 2. Experimental System The bench-scale cold model of an industrial-scale RSC is shown in Figure 1a based on geometric similarity modeling method. Two similarity criterions for RSC are required:13 (1) the geometric similarity ratio of the model to the prototype is about 1:8; (2) the gas Reynolds number Re in the inlet region is larger than the critical value 105. The schematic diagram of the experimental system is given in Figure 2. The air supplied from fan is injected from the top inlet, and the gas flow rate is controlled by valve 2 and valve 12. The purge valve 12 has been set to ensure the flow rate steady. The RSC has two cylinders, and the inner cylinder is assumed to be division membrane wall. The gas flow goes down along the inner cylinder from top inlet and flows circularly into the annular space when the gas flow reaches the water surface at the bottom. The gas will flow out from the top outlet which is designed at the outer cylinder. Furthermore, the schematic diagram of the industrial-scale RSC and the grid configuration of RSC are shown in Figure 1b and c. The Dantec hot-wire anemometry system was used to measure the gas flow velocity in the RSC. It was calibrated in standard wind tunnel beforehand with the relative error of (3%. The gas temperature maybe rises because of frictional heat, and the gas temperature has influence on measuring accuracy. So a thermometer was placed at the outlet of the RSC to keep realtime detection of the gas temperature change. The experiment
outer diameter of inner cylinder outer diameter of outer cylinder height of body bottom distance diameter of inlet diameter of outlet profile height
symbol
cold model RSC
industrial-scale RSC
Do1
0.30
2.30
Do2
0.50
3.70
H ∆H di do P1-P5
3.80 0.48 0.15 0.20 3.75, 2.95, 2.31, 1.59, 0.88
28.57 1.60 1.05, 0.90, 0.75 1.40 26.70, 20.93, 15.15, 9.38, 3.60
must be stopped when the temperature change is larger than 1 °C. The probe connected to the frame of the hot-wire anemometry has a single wire. The probe was fixed on the bracket, and the accurate measurement positions can be controlled by the probe bracket. A special data processing method was employed for calculating the mean velocity. The sampling frequency was set at 20 kHz and the sampling duration was 5s, and five measurements have been made for every point. Finally, the mean velocity of the every point is the average value of the five measurements. The details of measurement and calculation cases for the cold model are listed in Table 1. Measuring profiles P1-P4 and the outlet profile of inner cylinder P5 are located at different height cross sections of the cold model RSC and are shown in Figure 2. 3. Model Description and Boundary Conditions 3.1. Flow Field Simulation. To describe the flow field in an industrial-scale RSC accurately, turbulent flow model must be validated by experimental data. In the present work, the flow field of round confined jet flow in the cold-model RSC was investigated with the Spalart-Allmaras (SA) model, standard k-ε model, renormalization group (RNG) k-ε model, and realizable k-ε model under steady-state numerical simulation, respectively. The SA model is a relatively simple one-equation model that solves a modeled transport equation for the kinematic eddy viscosity. The SA model is designed for turbomachinery
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applications involving wall-bounded flow and has been shown to give good results for boundary layers subjected to adverse pressure gradients.17 The last three turbulent flow models through solution of two separate transport equations allow the turbulent velocity and length scales to be independently determined. The standard k-ε model is the simplest one proposed by Launder and Spalding for practical engineering flow calculations.18 The RNG k-ε model is similar in form to the standard k-ε model, but includes some refinements, e.g. the effect of swirl on turbulence is included in the RNG model which enhances the accuracy for swirling flows and recirculation flow.19 The realizable k-ε model is a relatively recent development and differs from the standard k-ε model in two important ways:20 (1) The realizable k-ε model contains a new formulation for the turbulent viscosity. (2) A new transport equation for the dissipation rate ε has been derived from an exact equation for the transport of the mean-square vorticity fluctuation. This model has been extensively validated for a wide range of flow, including round confined jet.12 For industrial-scale RSC modeling, the motion of a particle was described by the stochastic lagrangian multiphase flow model. The buoyancy force, gravity force, thermophoresis force, Brownian force, and Saffman’s lift force were all calculated in a lagrangian reference frame, given by dup g(Fp - Fg) ) FD(ug - up) + +F dt Fp
Figure 3. Principal sketch of the heat transfer and the gas-particle flow in the radiant syngas cooler. Table 2. Physical Properties of Ash Particle and Syngas Compositions
(1)
where F is an additional acceleration terms that includes thermophoresis force, Brownian force, and Saffman’s lift force/ unit particle mass, FD(ug - up) the drag force per unit particle mass and FD )
18µ CDRe Fpdp2 24
(2)
where, up is the particle velocity, ug the velocity of the fluid phase, CD the drag coefficient, FD the drag force of the particle, and dp the particle diameter. The relative Reynolds number Re defined as Re )
Fgdp |up - ug | µ
(3)
A fourth-order Runge-Kutta method was employed to predict the particle velocity and trajectories for the particle phase. The turbulent dispersion of particles was predicted by integrating the trajectory equations for individual particles, using the instantaneous fluid velocity, ug + u′, along the particle path during the integration. The values of u′ that prevail during the lifetime of the turbulent eddy were sampled by assuming that they obey a Gussian probability distribution: u′ ) ζ√u′2
(4)
where ζ is a normally distributed random number, and the remainder of the right-hand side is the local root-mean-square velocity fluctuations. Since the kinetic energy of turbulence is known at each point in the flow, these values of the root-meansquare fluctuation components can be obtained (assuming isotropy) as
√u′2 ) √V′2 ) √w′2 ) √2k/3
(5)
a
property
value
particle density particle heat capacity particle heat conductivity particle scattering coefficienta particle emissivity coefficienta ash flow temperature heat of fusionb syngas composition H2 CO CO2 H2S COS CH4 N2 NH3 H2O
2500 kg/m3 1450 J/(kg K) 1.87 W/(m K) 0.1 0.83 1/m 1255 °C 100 kJ/mol vol % 27.63 35.64 13.69 0.11 0.01 0.01 0.53 0.05 22.33
Data taken from ref 26. b Data taken from ref 28.
The particle-eddy interaction time dimension should not be larger than lifetime and size of a random eddy, respectively. And the two-way coupling method was applied to consider the interaction between the gas phase and the particle phase. 3.2. Heat Transfer Model. The primary function of the RSC is to cool the hot syngas from gasification process and to produce the high-temperature steam. So the syngas entering the RSC is at extremely high temperature, and radiation heat-transfer phenomena predominates a signification portion of the syngas cooler. But convective heat transfer effects cannot be ignored, especially for the place close to the membrane wall. Principal sketches of heat transfer and gas-particle flow near the membrane wall of RSC is presented in Figure 3. The discrete ordinate model (DOM)21 was used in present numerical model to solve radiative heat-transfer equations. This method uses radiative absorption and scattering coefficients for gas-particle radiative transfer. The syngas radiative coefficients were calculated by a weighted-sum-of-gray-gases model (WSGGM).22 The radiative properties of particle are related to the chemical compositions, particle size, particle shape, and surface roughness.23-25 The value of radiative coefficients of ash particle were estimated according to the results measured under a reduced environment by Mills and Rhine.26 For this work, the physical properties of ash particle and syngas composition are listed in Table 2.
Ind. Eng. Chem. Res., Vol. 49, No. 9, 2010 Table 3. Operating Conditions of Industrial-Scale RSC
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Table 4. Data of Ash Particle Size
variables
value
diameter (µm)
mass fraction (%)
typical particle diameter (µm)
syngas and particle inlet temperature inlet water temperature of membrane wall tube temperature of water in slag pool operating pressure inlet syngas volume flow rate inlet particle mass flow rate
1300 °C 313 °C 140 °C 4.0 MPa 10593.12 kmol/h 13243.7 kg/h
>2360 1180-2360 850-1180 425-850 150-425 75-150 45-75 e45
31.50 15.41 11.66 6.84 12.44 6.29 4.87 11.00
2360 1770 1015 638 288 113 60 30
The initial surface temperature of deposits Ta was estimated using heat flux balance equation: l)
λ(Ta - Tw) h(Tg - Ta) + σεp(Tg4 - Ta4)
× 103
(6)
where the deposit surface emisivity εp was assumed to be 0.83,26 and the convective heat transfer coefficient h was calculated based on the temperature gradient under the wall and flow conditions. The thermal conductivity λ of the deposit layer is a crucial model parameter, and the reference values from the experiments are 0.25 and 1.87 W/(m K) for solid ash and slag,26,27 respectively. The heat transfer between the gas phase and particle phase have also been considered by calculating the heat-transfer balance equation:
The standard wall function method18 was used to account for the near-wall effects in the flow field. The velocity correction was realized to satisfy continuity through the SIMPLE algorithm, which couples velocity and pressure. To evaluate the convective terms and turbulent kinetic energy, the second-order QUICK scheme was used. The PRESTO! scheme was used for pressure discretization, and the first-order upwind was used for other terms (turbulent dissipation rate, energy, and DOM, etc.). The solution of particle phase conversion equations was coupled with the gas phase. The simulations were performed using the Fluent program. A converged solution was defined when the global residual of variables
