On the effects of adding syngas to an ammonia-MILD combustion regime—A computational study of the reaction zone structure

Abstract

This study examines the reaction zone structure of syngas/NH3/CH4 blends in moderate or intense low oxygen dilution (MILD) conditions to determine the effects of variations in NH3 and syngas mole fractions upon the stability of the reacting system and NOx emission. A series of axisymmetric turbulent combustion simulations are performed using OpenFOAM and selected reaction mechanisms and, a modified version of the eddy dissipation concept. An increasing amount of syngas is added to the reacting mixtures of NH3/CH4 in MILD conditions. It is observed that a reduction in the NH3 mole fraction increases the OH radical concentration, which implies a reduction in the ignition delay. Further, for a given NH3 mole fraction, an increase in the syngas content of the fuel blend along with a reduction in CH4 leads to a rise in the process maximum temperature near the entrance zone. This reduces the flow temperature gradient, decreases the CH2O mass fraction, and boosts the stability of MILD combustion. Additionally, adding syngas to ammonia for MILD combustion decreases the reaction zone weakening, which is a major problem in MILD combustion of ammonia. In particular, blending syngas with NH3 and reducing the amount of CH4 in the fuel blend increases flame speed and heat generation. When syngas is added to ammonia MILD combustion, NOx and NxO decrease, and more complete combustion is achieved.