Blending effect between the natural gas and the liquefied petroleum gas using multiple co- and cross-flow jets on NOx emissions

A burner comprising multiple co- and cross-flow jets was developed to address the effect of blending the natural gas (NG) with the liquefied petroleum gas (LPG) on the NOx emissions from laminar/turbulent diffusion and triple flames in the normal and inverse configurations. Increasing the NG mole fraction in the blend from 0.0 to 1.0 decreased the effective Schmidt number from 1.4 to 0.7 thus displacing the flame envelope into higher velocity regions such that the corresponding decrease in residence time caused NOx reduction by 46%. While the prompt NOx increased by 11%, there was an additional NOx reduction by 238% due to the residence time decrease pertaining to the density/velocity difference. Conversely, increasing the LPG mole fraction from 0.0 up to 1.0 extended the flame stability limit from 8.2 to 12.7 m/s whereby the residence time decrease caused NOx reduction by 49%. Furthermore, the peak temperature reduction by the flame increased soot-radiation additionally caused exponential NOx reduction. The NG optimum mole fraction in the blend for reaching minimum NOx emissions of about 0.035 g/kW hr in the normal diffusion flame decreased from 0.80 to 0.25 as the firing rate increased 9.5 times, while a further NOx reduction to 0.028 g/kW hr was obtained in the inverse diffusion flame. Employing cross-flow opposing air jets reduced the NOx emissions further to 0.017 g/kW hr and shortened the flame by 36% as increasing the strain rate to 2900 s(-1) enlarged the peak turbulent kinetic energy to 11.9 m(2)/s(2) with 12 jets. In the triple flame mode, supplying the LPG across the fuel-rich wing and the NG across the lean wing combined high radiation efficiencies, flame shortening and extensive stability limits of 4.8 m/s. The NOx emission index thus reached 0.3 g/kg fuel for the inverse triple flames. (C) 2019 The Authors. Published by Elsevier B.V. on behalf of Faculty of Engineering, Ain Shams University.