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Full-scale CFD Simulation and Measurement in a Heat Recovery Steam Generator

Liang ZHAO, Lin MU and Hongchao YIN

School of Energy and Power Engineering, Dalian University of Technology, 116023, Dalian, China * E-mail: liangzhaoliangzhao@yeah.net

Received: 15 April 2013 /Accepted: 20 July 2013 /Published: 30 July 2013 Abstract: Flue gas generated by the wastewater incineration entrains a large number of ash particles which are composed of alkali substances into the heat recovery steam generator (HRSG). The deposition of particles rich in alkali on the tube surface of heat transfer can reduce the heat transfer efficiency; even cause severe capacity-limiting plugging and unscheduled shutdown. In the present work, a full-scale CFD model based on the Eulerian-Lagrangian scheme is implemented to simulation flue gas turbulent flow and heat transfer as well as the particle transport in the heat recovery boiler. Several User-Defined Functions (UDFs) are developed to predict the particle deposition or rebounding in the thermal boundary layer and detachment of deposited particles from the surface. On a basis of experiments, a new correlation of Young’s modulus is proposed to represent particle sticking behavior. The results of numerical simulation which includes deposition rates and deposition distributions with different sizes of particles in the heat recovery boiler show an acceptable agreement with the field measurements. Copyright © 2013 IFSA. Keywords: Ash particle deposition, Full-scale simulation, Sticking probability, Particle detachment, CFD simulation. 1. Introduction

In the production process of chemical industries, pharmaceutical industries, metallurgical industries, and pulp and paper industries, a great deal of waste water containing various chemical substances, such as liquid hydrocyanic acid, ammonium sulfate, lignin, aliphatic carboxylic acids and other organic and inorganic chemicals [1, 2] are generated as byproduct. Incineration method is a potential and effective treatment method for such high concentration and poor biodegradability organic wastewater [3]. Furthermore, in order to recover the waste heat after incineration, a heat recovery stream generator (HRSG) is installed at the tail of the waste water incinerator, and used to heat boiler feed water and produce steam. However, ash particles rich in

alkali (mainly Na) and with highly variable content and chemicals are also introduced and entrained into the heat transfer passage by the bulk gas transport.

Fly ash particles deposit and accumulate on the surface of tube banks, then lead to erosion and corrosion [4]. If soot blowers can’t remove the deposits designedly according to the actual HRSG operating conditions, in serious cases, these alkali vapors and sticky particles lead to ash bridges of slag flags at the narrow position of flue gas channel, even result in severe capacity-limiting plugging and unscheduled shutdown. So it is necessary to predict deposition trends quantitatively in order to assess required maintenance intervals.

The purpose of this work is to establish a CFD-based turbulent model combined with the several User Defined Functions (UDFs) for full-scale

Article number P_SI_415

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simulation of particle transport in high temperature flue gas and particle deposition/rebounding in the thermal boundary layer of tube banks. The input parameters such as size distribution, chemical composition are based on the field measurement and laboratory analysis.

2. Experimental Data The input parameters are derived from actual

operation conditions of the incineration furnace and heat recovery boiler in Acrylonitrile Plant at Sinopec Qilu Company Ltd.

A corner tube boiler is used for heat recovery and the tube banks are in a staggered arrangement with flue gas traversing across them. In high temperature gas flow passage the arrangements of tube bundles are in two ways based on the different transverse pitches. In the first half section of passage, the transverse pitch is 170 mm and the longitudinal pitch is 125 mm. Every four rows of tube bundles is one group and there are three groups altogether with a 551 mm distance between neighboring two groups. This space is applied as a working passage during overhaul or shutdown while in the second half part, the transverse pitch is 125 mm, and the longitudinal pitch is still 125 mm. This closer arrangement could increase heat transfer area in order to offset the insufficient heat transfer in the first half section due to large transverse pitches and working passages used. The specification of heat transfer tube is diameter 51 mm and wall thickness 3 mm.

125mm

51mm

Flue gas direction

Fig. 1. The two ways of arrangement of tube banks.

Usually chemical composition of ash deposit in the area of high temperature and low temperature has an acceptable approximation [5], while at the location where the flue gas temperature is lower than the dew point of alkali vapors, the ash samples could not be used owing to enrichment in these vapors. Deposit samples on the surface of heat transfer are collected through dust removal windows located on the furnace wall of the HRSG. And detailed information about major spices and chemical composition of ash deposits could be obtained by D/MAX-1200 X-ray diffractometer and RLX 3000 fluorescence spectrometer. Ash samples are collected at high temperature area (i.e. near the entrance to

heat recovery boiler, Tinflow=1123 K) and at low temperature area (i.e. neat the exit to heat recovery boiler, Toutflow≈600 K) respectively, see Table 1 and Table 2.

Table 1. XRD analysis of major species of ash deposits.

Sample Location Components Deposit near the entrance of HRSG

Fe2O3, α-Al2O3, Na7Al5Si5O24S3, Na2SO4, CaAl2O4, NaAlO2,

Deposit near the exit of HRSG

Fe2O3, FeO, α-Al2O3, NaAlO2, Na2SO4, FeSO4, CaAl2O4, NaAlO2,

Table 2. Chemical composition of ash deposits (wt %).

Sample Location

Deposit near the entrance of HRSG

Deposit near the exit of HRSG

CaO 2.11 1.24 MgO 0.14 0.84 Na2O 38.02 39.56 K2O 0 0.04

Al2O3 46.32 33.45 SiO2 8.57 8.47 TiO2 0.11 1.54 Fe2O3 4.73 14.86 Total 100 100

When fly ash particles with relative high temperature collide with the surface of heat transfer tubes, they could appear “abruptly cooling” phenomenon, and the temperature of ash particles reduces quickly and even below their melting temperature. Because of high viscosity, ash particles stick to tube surfaces, and deposits are formed. 3. Model Description

In the present work, an Eulerian-Lagrangian approach is implemented to afford the solution of particle-laden flue gas flow problems. The Eulerian model describes continuous phase behavior while the particle trajectory tracking is estimated by the Lagrangian model. Popular two equation turbulence model (realizable k-ε turbulent model) provides excellent representation of continuous phase turbulence in this simulation. For the wall-bounded internal flow simulated with the realizable k-ε turbulent model, the standard wall functions are used to link bulk flow conditions to the near-wall behavior in the wall adjacent cells in the computational domain. That is because in the full-scale industrial flow simulations, for the overall representation of turbulent flow and near-wall fluctuation, standard wall functions are economical and reasonably accurate to model the influence of the wall.

Since it’s intended to investigate the particle dispersion and deposition within a quite wide particle

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size range, it is necessary to take into consideration not only the drag force, but also the Brownian force, Saffman’s lift force and Thermophoretic force.

Under these assumptions, the governing equation of particle motion is given as:

Re1

24p D p TH

f p B L

du C Fu u F F

dt m (1)

The left side of the Eq. (1) represents the inertial

force per unit mass, m/s2, and in Eq. (1), uf and up is the continuous phase and particle velocity respectively, m/s, CD is the particle drag coefficient and expressed by Schiller & Nauman’s correlation, m is the mass of a particle, kg, Rep is the particle Reynolds number, FB and FL is the Brownian force and Saffman’s lift force [6, 7]. A modified equation of thermophoresis force as a function of Knudsen number which is developed by He and Ahmadi [7] by adjusting the amplitude and some parameters is applied to perform the thermophoresis in the heat recovery steam generator.

Since research into the deposition of ash particles arisen from the incineration of waste water rich in alkali is limited, a famous evaluation criterion of sticking probability which is proposed by Walsh et al. [8] is proper for the study of deposition mechanisms of waste water-incinerated ash particles. The viscosity of ash particles is calculated based on the viscosity-based empirical model developed by Browning et al. [9]. In this paper, a reference viscosity of 103 Pa s is applied due to a high fraction of alkali metal in ash particles. In the full-scale simulation of particle sticking the viscosity-based model is simple, but has a reasonable agreement with experimental data. Furthermore, the critical moment theory [10] is applied to predict the detachment of the deposited particles from the deposition surface. The critical wall shear velocity (m/s) is defined as:

2131

Ed

W

d

WCcu

p

A

p

Acr

, (2)

where Cc is the Cunningham correction factor, WA=0.039, ρ is the particle density, kg/m3, E is composite Young’s modulus, and defined as:

122 11

3

4

p

p

s

s

EEE

, (3)

where υs and υp is the Poisson’s ratio of the surface and particle respectively, Es and Ep is the Young’s modulus of surface and particle, Pa. Once the wall friction velocity of the turbulent flow exceeds the critical wall shear velocity, the particles will be removed from deposition surface. In order to predict the sticking feature accurately, a reasonable Young’s modulus which mainly depends on the physical properties of particles and deposition surface is

needed. However it is difficult to determine the Young’s modulus of the ash particles with complex chemical components. Thus an empirical correlation based on Ai and Fletcher [11] is developed in this paper, in which Tavg is the average temperature of particle and tube surface, K:

183 10 exp 0.03165p avgE T (4)

4. Simulation Conditions and Results A two-dimensional simplified calculation domain

is built up based on the actual size of waste heat steam generator; for the whole of domain the total volume is 1.5433×101 m3 with total cell numbers 227805 on a basis of grid independence test, as shown in Fig. 2.

Fig. 2. Computational grid generation.

Some UDFs are established to simulate the particle deposition and detachment in the thermal boundary layer as well as the physical properties of flue gas. The turbulent dispersion of particles tracked by Lagrangian method in the continuous phase is modeled by Discrete Random Walk (DRW) model associated with Random Eddy Lifetime. Three typical particle sizes 20 μm, 8 μm and 0.2 μm are used to perform the size distribution of ash particles with flow rates of 0.002 kg/s, 0.00005 kg/s and 0.0009 kg/s and the density of particle is 2600 kg/m3.

The temperature contour is shown in Fig. 3, and the outlet temperature is 686.2 K when a fixed wall

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temperature 490 K is specified. The overall heat transfer rate is 2461.61 kW, in which the radiation heat transfer rate is 1371.63 kW. Along with the heat transfer process and the reduction of temperature difference between the flue gas temperature and surface temperature, the heat transfer rate as well as radiation heat transfer in the every row of tube banks reduces gradually.

Fig. 3. Flue gas temperature contour at the inlet temperature of 1123 K.

Fig. 4. Deposition rate of 20 μm particles on the tube banks along with the flow direction.

Fig. 5. Deposition rate of 8 μm particles on the tube banks along with the flow direction.

Fig. 6. Deposition rate of 0.2 μm particles on the tube banks along with the flow direction.

When the calculation of flow field is converged, the particles are injected in the computation domain as a post-processing step to predict the particle fates. Once the particles reach the surface of tube banks, the UDFs of boundary layer start to execute to estimate whether particle deposition, rebounding or detachment. While the forces acting on the particles in different sizes have different influences on the particle motion. For dp=20 μm, due to large inertia, large particles become insensitive to Brownian force and Saffman’s lift force. These particles tend to flow their ballistic trajectories with shorter residence time in the heat recovery steam generator. Thus the particles of 20 μm can escape from the main flow field, penetrate the streamline with a large enough momentum normal to the surface of the tube and deposit on the surface initially. The results shows that most of large particles yielding to the inertial impaction deposit on the upwind side of heat transfer tubes in the first half part of heat recovery steam generator to cause an elliptical- or mountain-like-shaped deposits (see Fig. 4), while no particles deposit on the leeward side of tubes. The particles in the size of 8 μm and 0.2 μm are influenced by Brownian force and Saffman’s lift force significantly. Based on the numerical simulation, the intermediate-sized particles and submicron particles follow the main flow easily and have the longer residence time in the heat recovery boiler (see Fig. 5 and Fig. 6). The deposition locations of these particles are distributed more uniformly which is because some particles may deposit on the downstream side of tubes due to turbulent backflow phenomenon.

The temperature of particles that still transport in the heat transfer passage declines due to the convective and radiative heat transfer, then the texture of fly ash particles is harder than that in the high temperature area. Therefore, the adhesion probability reduces below 1, and some particles can rebound off from the surface after impaction or multi-impaction. On the other hand, the apparent structure of deposits on the front half part of tube banks are approximate melt conditions, while an off-white porous thin layer is formed in the tube banks near the exit to the heat recovery steam generator.

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4. Conclusions In this paper a developed particle deposition and

detachment model is proposed for the full-scale simulation of particle transport and deposition in a heat recovery steam generator. And in order to perform the accurate sticking behavior of particles when they reach the surface of tube banks, an empirical correlation of Young’s modulus is proposed accounting for the influence of particle and surface temperature. The results of numerical simulation are in an acceptable agreement with field measurements.

Acknowledgements

This work was financially supported by the Fundamental Research Funds for the Central Universities.

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