Influence of H₂ blending on NO_x production in natural gas combustion: Mechanism comparison and reaction routes

Hydrogen blending in natural gas is an effective way of carbon reduction. The effect of hydrogen blending on the reaction pathway of NO_x emission from natural gas combustion in industrial furnaces is studied by chemical kinetic calculations. To ensure the reliability of kinetic calculation, the NO_x prediction accuracy of the mechanisms GRI and PG2018 were compared by validation with corresponding experiments. It is found that the PG2018 mechanism is more accurate for NO_x prediction than GRI 2.11 and GRI 3.0, especially for N₂O-intermediate and NNH pathways. The effect of hydrogen blending on the chemical pathways of NO_x production was obtained based on PSR reactor and burner stabilized laminar flame with a well-controlled temperature profile. The results show that in the PSR reactor, N₂O-intermediate and thermal NO_x are predominating pathways for NO_x formation. NO_x emissions of unit fuel heating value first rise with the increase of blending ratio and then reduces rapidly while the blending ratio is greater than 50%, which is mainly due to the suppression of thermal NO_x and the increase of N₂O-intermediate NO_x. It can be found from calculation results of burner stabilized flame that the prompt NO_x decreases dramatically in the laminar flame and the NO_x production via NNH increases slightly as hydrogen blending ratio increases. With increasing hydrogen blending ratio, thermal NO_x is suppressed, and the N₂O-intermediate pathway is suppressed also at the flame sheet but enhanced in the flue gas due to the increment of the H₂O content. NO_x emission decreases with increasing hydrogen blending ratio in laminar flame at a given combustion temperature.