Advanced CFD Tools for Modeling Lean Premixed Combustion
in Ultra-Low NOx Burners in Process Heaters
Q. Tang, B. Adams, M. Bockelie, M. Cremer, M. Denison, C. Montgomery, A. Sarofim Reaction Engineering International
D. J. Brown
Stone & Webster, Inc.
Air quality emissions regulations, NOx in particular, are impacting the chemical process industry and forcing adoption of expensive selective catalytic reduction (SCR) units or retrofit of less expensive but relatively unproven ultra-low NOx burners. CFD modeling provides a potentially cost-effective method for evaluating NOx emissions and furnace performance for new burner technologies, thus minimizing furnace start-ups times and unforeseen performance impacts. However, existing CFD models are incapable of accurately predicting NOx emissions due to the complex geometries, turbulent mixing, lean premixed combustion chemistry, heat transfer and low NOx levels in the new generation of burners.
This paper reviews a new computational tool developed by REI specifically designed to model lean premixed combustion associated with low NOx burners in process heaters/furnaces. The new tool is based on a Hybrid Scalar Transport Probability Density Function Solver and includes adaptive mesh refinement to capture near-burner mixing, reduced chemical kinetic mechanisms with in-situ adaptive tabulation to compute finite rate chemistry, a combination of conventional Eulerian turbulence modeling with Monte-Carlo PDF methods to model turbulent reactions and mixing, and a matrix-free Newton-Krylov method to reduce solver computational time and improve robustness. Predicted flame characteristics and emission levels for low NOx burners in a test furnace will be discussed and compared with measured and observed data. 1. Introduction
Air quality emissions regulations resulting from the National Ambient Air Quality Standards (NAAQS) are requiring increasingly stringent ozone emissions from numerous furnaces in the chemical process industry. NOx emissions have been identified as a major contributor to ground level ozone, and have been targeted for significant reduction. These reductions are impacting a large number of industrial boilers and heaters in non-attainment areas, approximately 1900 in the Houston-Galveston area alone, for example, and are forcing furnace operators to adopt expensive selective catalytic reduction (SCR) technology (an SCR retrofit can exceed $3M for a large ethylene cracking furnace) or retrofit less expensive but relatively unproven ultra-low NOx burners. These ultra-low NOx burners can provide a relatively inexpensive solution to NOx reduction requirements, but reductions must be achieved without compromising the heat release profiles or flame ‘quality’ of the burners: it is not uncommon for ultra-low NOx burners to have wider and longer flames than conventional burners or even, in the worst case, for flames to impinge directly on the heat absorbing process tubes. CFD modeling provides a potentially cost-
Advanced CFD Tools for Modeling Lean Premixed Combustion in Ultra-Low NOx Burners in Process Heaters AFRC-JFRC Joint International Symposium 2004
effective method for evaluating NOx emissions and furnace performance for new burner technologies, thus minimizing furnace start-ups times and unforeseen performance impacts. However, existing CFD models have shown poor accuracy in predicting furnace NOx emissions due to the combination of complex geometries, turbulent mixing, lean premixed combustion, and low NOx levels in the new generation of low NOx burners.
Existing CFD software has been used to study more general process furnace characteristics such as furnace exit temperatures, heat...