(552d) The Relative Contribution of Particle and Gaseous Radiation in Oxy-Fuel Combustion

Radiation is the main mechanism for heat transfer in a combustion chamber and it is therefore essential to have a good understanding of it for the design of heat transfer surfaces. The changed conditions of oxy-fired, compared to air-fired boilers, have important effects on the radiative characteristics of the combustion gas. The increased concentrations of carbon dioxide and/or water vapor, depending on whether dry or wet flue gases are recycled, results in an enhanced contribution to emission and absorption of thermal radiation. The impact of these changes depends not only on increased molar fractions of the gaseous species, but also on the relative importance of gaseous radiation compared to particle radiation and how these two interact. In a coal-fired boiler, particles in the form of fuel, soot and ash contribute to the emitted radiation and can in the flame zone dominate the radiative heat transfer. The radiative properties of particles are characterized by relatively small spectral variations and can to a large extent be treated as gray, while the properties of the combustion gases show a complex spectral behavior, which has significant effects on the radiative properties. Due to these differences there is a need to investigate both gas and particle radiation in oxy-fired atmospheres, to be able to quantify changes compared to air-fired units. Such knowledge is of large value in the scaling of the oxy-fuel process, in which boilers of industrial scale will be designed, based on studies in lab and pilot scales and previous experience of air-fired facilities.

This work focuses on radiation in oxy-fired conditions. Both gas and particle radiation is modeled in an axi-symmetric cross section of a cylinder and differences between air- and oxy-firing are investigated. The intensity field is calculated according to the discrete transfer method by solving the intensity along a number of rays that crosses a radial profile. Scattering along the rays is related to the distance to the centre of the cylinder. For the gas radiation, a Statistical-Narrow-Band (SNB) model is applied as reference while the particle radiation is modeled by empirical correlations accounting for spectral properties of coal particles. Scattering of the particles is assumed to be isotropic. Included in the analysis is also a non-gray or banded formulation of the Weighted-Sum-of-Gray-Gases (WSGG) model and grey approximations. These two approaches are computationally efficient and reasonable to use in CFD-simulations. A grey approximation is the most simplified approach and probably the most common way to handle gas radiation in CFD-modeling of combustion. The banded formulation of the WSGG model is an effective method to increase the accuracy of the radiative source term at a relatively low computational cost.

The investigated cases cover both air-and oxy-fired conditions and the properties of the combustion gas are based on measured data from a lignite flame in Chalmers 100 kW rig. Temperatures spans from 1000 K to 1800 K and gas concentrations corresponding to both dry and wet flue gas recycling are examined. The particle load is obtained by fitting modeled radiation intensity profiles against intensity profiles measured by a narrow angle radiometer. Wall fluxes and the radiative source term along the cylinder diameter are compared to evaluate differences in the radiative heat transfer between air- and oxy-fuel combustion. Included in the study is also a sensitivity analysis of the influence of particle load and scattering. The different approximations for gaseous radiation are evaluated to quantify the errors they result in when they are applied in conditions with a high particle load. Scale effect are also addressed by increasing the diameter of the cylinder while maintaining the same profiles of temperature, gas species and particle concentration.