Exploring Computational Methods for Predicting Pollutant Emissions and Stability Performance of Premixed Reactions Stabilized by a Low Swirl Injector

ABSTRACT

This article addresses the numerical modeling of NO_x emissions and lean blowoff (LBO) limits of confined and pressurized turbulent premixed flames stabilized with a low swirl burner. The study also evaluates existing numerical methods that can be used to predict exhaust pollutant emissions and reaction instability close to the LBO limit. One of the strategies presented in the article consists of establishing a chemical reactor network (CRN), which is a simplified model of the fluid dynamics and energy balance of the system coupled with a detailed reaction mechanism. Since the computing turnaround time for a CRN model is several orders of magnitude less than the simplest computational fluid dynamics (CFD) case, the parameters controlling the pollutant formation and stability of the system can be quickly assessed over the complete flammable range. The results show the value of a simple reactor network as a design tool that can be used to optimize emissions and LBO limits of combustion systems. The parametric analysis examines the most important variables that control the formation of pollutant species (pressure, recirculation of gases, heat losses, geometry variables, air to fuel ratio, and fuel composition). Experiments were carried out at a pressure of 304 kPa using two fuel compositions: natural gas and natural gas enriched with up to 90% hydrogen (by volume). The results obtained with the CRN for NO_x and LBO are in good agreement with those observed experimentally and show that, for ultra-low NO_x burners, the lowest NO_x emissions are concomitant to the onset of instabilities associated with LBO. In sharp contrast, the CFD-based simulations of NO_x fail to accurately predict the effect of the fuel composition on the LBO limit. Further analysis of the CRN results show that, at lean conditions and pressures above three atmospheres, NO_x is formed primarily through the N₂O pathway, regardless of the fuel composition. The CRN model indicates that adding hydrogen to natural gas promotes the production of NO_x through the Zeldovich, NNH, and N₂O routes while reducing formation via the prompt route. Understanding the relative contributions of each route provides a starting point from which to propose modifications to the system to reduce NOx emissions.

KEYWORDS:

• Fuel flexibility
• Hydrogen
• Lean blowoff
• NO_x
• Pollutant simulation

Acknowledgments

The experimental results provided by David Beerer and the CFD analysis conducted by Mathias Neumayer are highly appreciated. Discussions with Phil Malte, John Kramlich, Megan Karalus, and Igor Nossolev regarding the application of the CRN were very helpful.

Funding

The authors gratefully acknowledge Colciencias for the financial support of Andres Colorado through the scholarship Francisco Jose de Caldas. Also, the support of the California Energy Commission (Contract 500-13-004) is gratefully appreciated.

Additional information

Funding

The authors gratefully acknowledge Colciencias for the financial support of Andres Colorado through the scholarship Francisco Jose de Caldas. Also, the support of the California Energy Commission (Contract 500-13-004) is gratefully appreciated.