# CFD analysis of the combustion process in a boiler of a 160 MWe power plant: leakage influence

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## Abstract

This paper presents a developed and validated CFD model of a boiler with the objective of studying a presumed defective actual operating condition, that is, air leakage into the furnace. Three cases were assessed, concerning the amount of air leakage across the boiler water seal. The base case considered the actual operational condition of 5% air leakage. Case 1 assumed 10%, and Case 2 simulated a more extreme leakage of 25%. The influence of the air leakage was analyzed with emphasis on temperature profiles, flow, wall heat flux, O_{2}, CO, NOx, Soot and CO_{2} formation. Eddy dissipation together with finite rate chemistry models were used to model the combustion process, while the *k*-*ω* approach was considered to predict the turbulence effect. Brazilian pulverized coal type CE3100 was adopted as fuel, burning in atmospheric air. The conservation equations of mass, momentum, energy and chemical species were solved by Finite Volume Method using Ansys CFX commercial software. As main results, it has been observed that air leakage across the water seal has influence over the profiles of temperature and concentration of oxygen and carbon dioxide. Significant amounts of air leakage can induce the plant staff to change the amount of air to be fed through the burners, changing the burning condition, thus impairing energy generation. Although the simulated case concerns a specific operational condition of a given thermal power plant, the results demonstrate the capability of CFD as a decision support tool. It has been found that even a small drop in the flue gas temperature in the combustion chamber due to air leakage in the water seal significantly affects the radiation heat transfer from flue gases to the chamber walls. The overall reduction in average wall heat flux was − 0,3% and − 1,81% of base case for cases 1 and 2, respectively. This situation alters the boiler behavior related to the formation of pollutants, especially NOx, also reduces the burning efficiency, increasing the concentration of CO and CH_{4} in the exit region.

## Keywords

Coal combustion CFD Leakage Water seal Thermal power plant## List of symbols

- \({\text{NO}}_{x}\)
Oxides of nitrogen

- \({\text{CH}}_{4}\)
Methane

- \({\text{O}}_{2}\)
Oxygen

- \({\text{N}}_{2}\)
Nitrogen

- \({\text{CO}}_{2}\)
Carbon dioxide

- \({\text{CO}}\)
Carbon monoxide

- \({\text{H}}_{2} {\text{O}}\)
Water vapor

- \({\text{NH}}_{3}\)
Ammonia

*k*Constant of chemical reaction rate; or turbulent kinetic energy, m

^{2}/s^{2}*x*Spatial coordinate, m

*r*Vector position or Particle radius

*s*Vector direction, m

- \(S^{{\prime \prime }}\)
Radiation source term, W/m

- \(K_{\alpha }\)
Absorption coefficient, m

^{−1}- \(\tilde{U}\)
Average velocity, m/s

- \(Sc\)
Schmidt number

- \(C_{\mu }\)
Empirical turbulence model constant

- \(C_{\text{o}}\)
Mass fraction of raw coal, kg/kg

- \(C_{\text{ch}}\)
Mass fraction of char, kg/kg

- \(p^{*}\)
Modified pressure, Pa

- \(P_{\text{a}}\)
Atmospheric pressure, Pa

- \(\bar{p}\)
Average pressure, Pa

*D*Dynamic mass diffusivity, m

^{2}/s- \(\tilde{Y}\)
Average mass fraction kg/kg

*Y*_{1},*Y*_{2}Mass Fractions

*I*Total radiation intensity, W/m

^{2}- \(\bar{R}\)
Chemical reaction rate, kg/(s.m

^{3}) or rate of formation/destruction of chemical species, W/m^{3}.kg- \(\Re\)
Universal ideal gas constant 8314.5 kJ/(kmol K)

*E*Activation energy, J/kmol

*A*Empirical coefficient, (m

^{3}/s)/kmol- \(\tilde{C}\)
Average molar concentration, kmol/m

^{3}- \(\overline{MM}\)
Molecular mass, kg/kmol

- \(k_{1}\)
Empirical constant

- \(k_{2}\)
Empirical constant

- \(\tilde{h}\)
Average enthalpy of mixture, kJ/kg

- \(h_{\alpha }^{0}\)
Enthalpy of formation, kJ/kg

**t**Time, s

- \(c_{\text{p}}\)
Specific heat, kJ/(kg.K)

*S*Path length, m; or source term, W/m

^{3}*n*Reaction order

## Greek symbols

- \(\sigma_{k}\)
Prandtl number

- \(\sigma_{\varpi }\)
Prandtl number

- \(\tau_{w}\)
Shear stress in the wall, Pa

- \(\rho\)
Density, kg/m

^{3}- \(\mu\)
Dynamic viscosity, N s/m

^{2}- \(\varepsilon\)
Dissipation of turbulent kinetic energy, m

^{2}/s^{3n}- \(\beta\)
Temperature exponent or empirical constant

- \(\beta^{{\prime }}\)
Empirical constant

- \(\alpha\)
Empirical constant or α-th chemical species

- \(\varPi\)
Product symbol

- \(\gamma\)
Concentration exponent

- \(\eta\)
Stoichiometric coefficient, kmol

- \(\kappa\)
Thermal conductivity, W/(m K)

- \(\sigma\)
Stefan-Boltzmann constant, 5.678 × 10

^{−8}W/(m^{2}K^{4})- \(\delta\)
Krönecker delta function

## Subscripts

- j
Index

- i
Index or Chemical species

- k
Chemical reaction or index

- t
Turbulent

- rad
Radiation

- rea
Chemical reaction

- g
Gas

- s
Surface

- d
Oxygen diffusion

- o
Raw coal

- ch
Char

- ref
Reference

- p
Products or particles

- eff
Effective

## Notes

### Acknowledgements

The authors thank for the support of MCT/CNPq—National Research Council (Brazil).

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