Numerical modeling of the effect of aerodynamics on NO_x emissions and char burnout for combustion of coal in O₂/CO ₂

Combustion of coal in O₂ and Recycled Flue Gas (RFG) medium is one of the approaches to obtain pure CO₂ stream from an existing power plant that can be sequestered to reduce the greenhouse gas emissions into the atmosphere. Other advantages of this approach are an increase in char burnout and reduction in NO_x emissions. However, in order to retrofit the existing boiler, approximately 30% O₂ and 70% CO₂ blend is required in the oxidizer stream. This leads to a decrease in the volume of combustion gases which subsequently changes the mixing pattern of oxidizer and coal particles inside the boiler. If coal particles and oxygen are not well mixed, NO_x emissions will be altered due to local fuel rich pockets. In the present paper, an, axi-symmetric, 2-D computational model was developed using Fluent CFD code, for a 1,000 lb steam/hr "A-frame", water-tube research boiler to identify and optimize the key variables that influence the NO_x emissions. Model predictions were compared with the experimental measurements of gas temperature, particle speed and gaseous emissions for Upper Freeport (Bituminous) coal fired in air medium. The effects on gaseous emissions such as NO_x, and CO₂ due to change in combustion gas loading and presence of increased CO₂ are predicted under similar operating conditions. The effect of swirl number was analyzed to minimize the NO_x emissions from the boiler in enriched O₂/CO ₂ medium.