Numerical Simulation Study of the Influence of Flue Gases Recirculated on NOx Emissions

Abstract

Flue gas recirculation is considered to be one of the most promising and cost-effective NOx reduction strategies for combustion systems. Therefore, it has great scientific and practical significances to investigate the flue gas recirculation process. Fluent was used to perform a numerical flue gas recirculation simulation on natural gas combustion in a burner, revealing the relationship between NOx emission and circulation ratio, circulation temperature, and circulation locations. The following conclusions were drawn: the NOx generated with gas recirculation on is less than when there is no gas recirculation; the lower the gas circulation temperature, the less the NOx; when the circulation is 25%, the NOx generated is low; less NOx is generated if the gas is firstly mixed with the air before combustion. Combustion processes are typically considered as ones of the main factors responsible for the emission to the atmosphere of important air pollutants, such as NOx, SO2 and particulate matter. NOx is produced during the combustion process in a high temperature region and is known as a source of major air pollution that generates ozone when it reacts with sunlight; NOx also causes respiratory diseases (1-3) . Flue gas recirculation (FGR) is an interesting technique for reduction of nitrogen oxide (NOx) emissions that can be applied to boilers or internal combustion engines. This technique includes recirculation of part of flue gases back to the combustion chamber; it may be responsible for various effects on combustion, pollutant formation and emissions. CFD is an analysis of systems involving fluid flow by means of computer-based simulations. These systems may also involve heat transfer and associated phenomena such as chemical reactions. CFD simulations are based upon a numerical solution of basic equations of the fluid dynamics, conservation of mass, momentum, and energy, together with mathematical sub-models. The equations can be solved in three-dimensions. Comprehensive modelling of combustion in general requires simulation turbulent fluid dynamics, chemical kinetics as well as their interactions. FLUENT 15 is a general-purpose CFD code, which is based on finite volumes, and the region of interest is divided into small sub-regions called control volumes. The equations are discretized and solved iteratively, providing the value of each variable (velocity, temperature, mass fractions etc.) for each control volume throughout the calculation domain. NOx emissions from natural gas combustion are studied, with the objective to demonstrate the applicability of stationary computational fluid dynamics simulations, including a detailed representation of gas phase chemistry, to a burner fired reactor using natural gas as a fuel.