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Mechanisms and strategies for ash deposition reduction in flue gas heat exchanger

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Abstract

Ash deposition in flue gas heat exchanger affects its heat transfer performance and lifecycle, which becomes a crucial factor restricting the efficient recovery and utilization of flue gas waste heat. In this paper, a numerical method was established to investigate the characteristics of ash deposition in tube bundle heat exchangers. An integrated fouling model including transport, rebound, deposition, and removal of particles was employed to predict the behaviour of particles. Then, the effects of particle diameter and flue gas velocity on collision mass, deposition mass, absolute deposition ratio, and relative deposition ratio were studied. At last, the thermal–hydraulic and ash deposition characteristics of three different tube shapes were compared. The results showed that the low-velocity regions on the circumference were the primary locations of particle deposition, and the medium-diameter particles were the main deposition components. With the increase in flue gas velocity, the deposition mass of large-diameter particles decreased, and that of small-diameter particles increased. The use of an elliptical tube and flattened round tube with an apex angle of 60°had also excellent thermal–hydraulic and anti-fouling performances. Consequently, the ash deposition and wear can be reduced by increasing flue gas velocity, filtering medium and large-diameter particles, and using elliptical and flattened round tubes.

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Abbreviations

c p :

Specific heat [j/(kg K)]

C m :

Mass factor

C u :

Cunningham correction factor

d p :

Particle diameter (m)

D :

Tube diameter (m)

D ω :

Cross-diffusion term

e :

Restitution coefficient

E :

Young’s modulus (Pa)

E * :

Equivalent Young’s modulus (N/m2)

E n :

Normal adhesion energy (J)

E t :

Tangential impact energy (J)

F :

Contact load (N)

F ad :

Additional force on particles (N)

F el :

Contact load under the limiting elastic deformation (N)

g :

Gravitational acceleration (m/s2)

G k :

Production of turbulence kinetic energy

G ω :

Generation of ω

G * :

Effective shear modulus (N/m2)

h el :

Distance approach under the limiting elastic deformation (m)

k :

Turbulence kinetic energy (J)

K :

Composite Young’s modulus (N/m2)

L d :

Downstream length (m)

L u :

Upstream length (m)

m * :

Effective mass

m c :

Collision mass (kg)

m d :

Deposition mass (kg)

n·∇U :

Velocity gradient of the gas

Nu f :

Nusselt number

p :

Pressure (Pa)

p max :

Maximum pressure at the centre (Pa)

P l :

Longitudinal pitch (m)

Pr :

Prandtl number based on the average temperature of the flue gas

Pr w :

Prandtl number based on the wall temperature

P t :

Transverse pitch (m)

Q A,a :

Surface adhesive energy (J)

Q A,r :

Adhesion energy (J)

Q e :

Elastic energy (J)

Q el :

Limiting elastic energy (J)

Q k :

Kinetic energy (J)

Q p :

Energy loss (J)

Q pe :

Elastic energy stored in the plastic deformation area (J)

r e :

Contact radius (m)

r el :

Contacting radius under the limiting elastic deformation (m)

R * :

Effective radius

R 1 :

The radius of incident particles (m)

R 2 :

The radius of target particles (m)

R c :

Critical contact radius (m)

Re :

Reynolds number

S :

Area of the tube (m2)

T :

Temperature (K)

t :

Time (s)

u :

Gas-phase velocity (m/s)

u i :

Velocity component in x and y directions (m/s)

u j :

Local velocity component (m/s)

u p :

Particle velocity (m/s)

u tc :

Critical wall shear velocity (m/s)

v i,l :

Limiting elastic velocity (m/s)

v i,n :

Normal velocity of the incident particle (m/s)

v r :

Rebound velocity of the particles (m/s)

v i,t :

Tangential velocity of the incident particle (m/s)

v w :

Local wall friction velocity (m/s)

x i :

The axis of x and y (m)

y :

Elastic load limit (N)

Y :

Yield stress (N/m−2)

z :

Number of the tube rows

α :

Location of the tube surface (°)

β :

The effective coefficient of the contact radius

Г :

Surface energy (J/m2)

ε n :

A correction factor of the tube row

θ :

Apex angle (°)

θ cr :

Critical collision angle (°)

λ :

Thermal conductivity [W/(m K)]

μ :

Dynamic viscosity (N s/m2)

μ t :

Turbulent viscosity (N s/m2)

μ * :

Effective friction coefficient

ν :

Kinematic viscosity of the gas (m2/s)

ξ :

Friction factor

ρ g :

Density for the gas (kg/m3)

ρ p :

Density of the particle (kg/m3)

σ k :

Turbulent Prandtl number for k

σ ω :

Turbulent Prandtl number for ω

τ r :

Particle relaxation time

υ :

Poisson’s ratio

χ :

A factor that is related to the tube pitch

ω :

Specific dissipation rate

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Acknowledgements

This work has been supported by the National Key Research & Development Program of China (2018YFE0108900) and the project LTACH19033 “Transmission Enhancement and Energy Optimized Integration of Heat Exchangers in Petrochemical Industry Waste Heat Utilisation”, under the bilateral collaboration of the Czech Republic and the People’s Republic of China (partners Xi’an Jiao Tong University and Sinopec Research Institute Shanghai; SPIL VUT, Brno University of Technology and EVECO sro, Brno), programme INTER-EXCELLENCE, INTER-ACTION of the Czech Ministry of Education, Youth and Sports. Declarations

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Authors and Affiliations

  1. Key Laboratory of Thermo-Fluid Science and Engineering, Ministry of Education, Xi’an Jiaotong University, Xi’an, 710049, Shaanxi, China

    Zefeng Guo, Nianqi Li, Qiuwang Wang & Min Zeng

  2. Sustainable Process Integration Laboratory - SPIL, NETME Centre, Faculty of Mechanical Engineering, Brno University of Technology - VUT Brno, Technická 2896/2, 616 69, Brno, Czech Republic

    Jiří Jaromír Klemeš

Authors
  1. Zefeng Guo
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  2. Nianqi Li
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  3. Jiří Jaromír Klemeš
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  4. Qiuwang Wang
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  5. Min Zeng
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Corresponding author

Correspondence to Min Zeng.

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Guo, Z., Li, N., Klemeš, J.J. et al. Mechanisms and strategies for ash deposition reduction in flue gas heat exchanger. Clean Techn Environ Policy 24, 77–93 (2022). https://doi.org/10.1007/s10098-021-02083-2

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  • DOI: https://doi.org/10.1007/s10098-021-02083-2

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