Numerical characterization of a novel staged combustion concept applied to OXY-fuel carbon capture technology

In oxy-combustion the fuel is burnt in a mixture of oxygen and recirculated flue gas to keep the temperature inside the furnace to levels similar to conventional combustion. This eliminates the atmospheric nitrogen from the process, leading to a flue gas consisting mainly of carbon dioxide and water vapor. Further on, the CO2 can be separated for storage purposes. A major drawback of the conventional oxy-fuel combustion technology consists in the high amount of flue gas that has to be recirculated in order to control the temperature level inside the furnace. A novel oxy-fuel firing concept based on a combination of pulverized coal burners operating under non-stoichiometric conditions is investigated as a solution for lowering the necessary flue gas recirculation rate, while keeping the temperature inside the furnace at feasible levels. This paper presents a numerical analysis of the most relevant aspects for this new firing concept, such as process specifics and limitations, burner design criteria, aerodynamic characterization of the near burner zone, flame ignition and temperature. First the process is defined via thermodynamic calculations which are necessary to establish the operating conditions and to generate sets of parameters for the design phase of the burners. Subsequently the parameters generated in the first phase are used as boundary conditions for the design of the burners via CFD simulations. The CFD code used in this study is updated for oxy-firing conditions with the recent developments in terms of gas phase reactions, char conversion modeling and radiative heat transfer in high temperature atmospheres with elevated CO2 concentration. Additionally, the most relevant aspects regarding the validation of the CFD code against in-flame experimental values are presented and discussed. The simulations show good agreement with the averaged experimental data collected along the flame centerline.