Research On Natural Gas Low NO_x Combustion Based On The Flue Gas Recirculation For Industrial Boiler

To obtain the goal of less than 30 mg/Nm3 for NO_x emission in the natural gas-fired boiler,a fuel-staged low-NO_x burner was studied and developed based on external flue gas recirculation(FGR).Through experiments on the 1.4 MW pilot scale gas-fired boler and compution fluid dynamics(CFD)simulation,the effects of fuel-staged,equivalence ratio and FGR ratio on the NO_x emission,flame stability and burn-out were studied.A scaled-up criterion for the gas-fired burner was also developed based on the NO_x emission control and flame structure,and was verified on a 7 MW and 10.5 MW demonstration hot water boiler.The experimental results on the 1.4 MW pilot scale gas-fired boler showed that fuel-staged and FGR are the main factors to reduce the NO_x emission.With the increase of secondary fuel ratio,the NO_x emission are increased firstly,and reached the peak value of 135 mg/Nm3 when the secondary fuel ratio is 50%,and subsequently decreased.Excess air coefficient and secondary fuel ratio have synergistic effect on NO_x emission.For the effect of FGR,the reduced flame temperature significantly inhibited the formation of thermal NO_x,which is priority than the reduced oxygen pressure.And FGR can also reduce the prompt NO_x formation.With inceasing of the FGR ratio up to 10%,the NO_x emission is sharply reduced,where 60-65% of NO_x emission is cut down at 10% of FGR ratio.Howerver,continually increase FGR ratio,the effect became weak on NO_x formation.With the help of primary swirl flame,the flame stability also is affected by the FGR ratio and secondary fuel ratio.There showed a fan-shaped region with two boundaries formed by extinction limit of primary and secondary flame due to the reduced oxygen pressure in the combustion air stream mixing with recycled flue gas.An CFD model with FLUENT code was studied according to experimental data of a swirl burner at Sydney university and verified by the results of 1.4 MW pilot scale experiments.The results showed a CFD model including realizable k-epsilon turbulence model,EDC combustion model,JL-2 reaction mechanism,NO_x generation combined with 5-step reaction can meet the requirement of burner desin.The results of CFD simulation furtherly explained the effects of FGR and fuel-staged on distribution of velocity,temperature,mixture fraction and NO_x formation rate in the furnace.A scaled-up criterion for the gas-fired burner was also developed based on the NO_x emission control and flame structure with the help of CFD simulation,which is different from the criterion based on heat output.And then the 1.4MW burner is optimized and scaled up to 7MW and 10.5MW for demonstration project.The demonstration results verified the scale-up criterion,and the NO_x emissions are also lower than 30 mg/Nm3.