NO reduction and NO2 emission characteristics in rich-lean combustion of hydrogen

Abstract

Hydrogen is a clean alternative to conventional hydrocarbon fuels, but it is very important to reduce the nitrogen oxides (NOx) emissions generated by hydrogen combustion. The rich-lean combustion or staged combustion is known to reduce NOx emissions from continuous combustion burners such as gas turbines and boilers, and NOx reduction effects have been demonstrated for hydrocarbon fuels. The authors applied rich-lean combustion to a hydrogen gas turbine and showed its NOx reduction effect in previous research. The present study focused on experimental measurements of NO and NO 2 emissions from a co-axial rich-lean burner fueled with hydrogen. The results were compared with diffusion combustion and methane rich-lean combustion. Significant reductions in NO and NO 2 were achieved with rich-lean combustion. The NO and NO 2 reduction effects by rich-lean combustion relative to conventional diffusion combustion were higher with hydrogen than with methane.Keywords: Hydrogen, Rich-Lean Combustion, Staged Combustion, Nitrogen Oxides, Zel'dovich NO, Fenimore NO, prompt NO, NO 2 INTRODUCTIONHydrogen is considered an ideal alternative to conventional hydrocarbon fuels, because hydrogen can be produced from any kind of energy source and as it burns without emitting soot or carbon dioxide. Nitrogen oxides (NOx) are the only potential pollutants from hydrogen combustion, and it is crucial to reduce NOx emissions from hydrogen combustion.Rich-lean combustion or staged combustion is known as a technique to reduce NOx formation in continuous combustion burners such as gas turbines and boilers. The effect has been demonstrated with conventional hydrocarbon fuels by many researchers [1][2]. The authors have applied rich-lean combustion to a hydrogen gas turbine and shown its effect of NOx reduction in previous research [3][4]. The rich-lean combustion reduces the thermal NO, the Zel'dovich NO [5][6], which is the main part of combustion-generated NOx. At the same time, the prompt NO, Fenimore NO [7], is formed in hydrocarbon combustion especially under fuel-rich conditions. Though the amount of prompt NO is quite small for conventional premixed or diffusion combustion, it could become a significant portion of the total NO emissions from rich-lean combustion due to the largely decreased thermal NO and the existence of fuel-rich mixture. Also, the NOx emissions from combustion include NO 2 which is more harmful to human health than NO and also has a strong green house effect. The current paper analyzes the emission characteristics of NO and NO 2 from a coaxial rich-lean burner fueled with hydrogen. The results are compared with rich-lean combustion with methane.

Performance comparison between hydrogen and gasoline fuelled S.I. engine

“…The wide flammability of hydrogen also permits hydrogen engine fuelled to operate at lean and very lean mixtures and to obtain an improvement in engine thermal efficiency [13,14], without an important cyclic variation.…”

Using Hydrogen Influences to Control the Greenhouse Gas Emissions from Methane-Hydrogen Turbulent Flame

“…Assuming a unit Lewis number, the enthalpy equation is given by ∇. (ρ h) = ∇ • ( μ e σ h ∇h) + μ e σ h 1 r ∂h ∂r + q rad (8) in which the turbulent Prandtl numbers σh is assumed to be 0.85. Thermal radiation of gases is taken into account assuming an optically thin flame.…”

“…Literature also shows that there is less efforts to pay attention to investigate the hydrogen combustion itself. Alternatively, such studies have mainly focused on NOx emissions from such flames [5][6][7][8]. So, there is the lack of knowledge in area of the blended fuels combustion and their greenhouse gases emissions.…”

Numerical study of hydrogen effects on pressurized methane‐oxy combustion in counter‐flow diffusion flame

“…In theory, unlike hydrocarbon‐based fossil fuels, hydrogen is a clean fuel that does not emit greenhouse gases (such as CO 2 ) or have dust‐emitting sources (such as NO x and SO x ) in the energy‐generation process. However, during the combustion process, when a nitrogen‐containing oxidant (air) is used, nitrogen oxides (NO x ) are generated because of the high temperature of the hydrogen flame 1 . In the combustion system using hydrogen because carbon does not theoretically exist, fuel NO x and prompt NO x are not produced; however, the nitrogen present in the oxidant reacts at the high temperature to generate thermal NO x.…”