Influence of Inlet Dilution of Reactants on Premixed Combustion in a Recuperative Furnace

Abstract

This paper presents a numerical study by RANS modeling that investigates the effect of external dilution on the premixed combustion occurring in a recuperative furnace. Calculations are performed using the detailed GRI-Mech 3.0 mechanism to ensure the accuracy of the modeling. Results of the in-furnace flow, temperature, and concentrations of OH, O 2 , CO 2 and NO x are provided. It is found that the external dilution with the inert gas CO 2 plays a significant role in establishing the premixed MILD (Moderate or Intense Low-oxygen Dilution) combustion. Externally diluting the reactant mixture not only reduces the initial concentration of O 2 but also ensures a stronger internal dilution by recirculation of more hot combustion products. Importantly, the latter effect is more significant for achieving the MILD regime. There is a critical mass fraction of the diluent CO 2 present, below which MILD combustion cannot occur. While the traditional premixed flame produces much more NO x than the MILD combustion, the emission of NO x appears to result most from the thermal-NO route and least from the N 2 O route no matter which mode occurs. Moreover, the present simulation demonstrates that the MILD mode occurs over a wider range of initial reactant conditions for premixed combustion than for diffusion combustion.

Flameless combustion role in the mitigation of NOXemission: a review

“…However, chemical effects, such as the addition of H 2 O and CO 2 to NO emission, were observed. The results showed that the reduction in the temperature of beak flame with additional CO 2 is higher than that of the beak flame with additional H 2 O . Diluted fuel (50% natural gas and 50% N 2 ) had an effect on industrial furnaces in highly preheated and diluted air (1273 K with 10% O 2 and 90% N 2 ) combustion.…”

“…Influence of diluents on temperature uniformity [51]. temperature of beak flame with additional CO 2 is higher than that of the beak flame with additional H 2 O [98][99][100][101]. Diluted fuel (50% natural gas and 50% N 2 ) had an effect on industrial furnaces in highly preheated and diluted air (1273 K with 10% O 2 and 90% N 2 ) combustion.…”

Reaction Zones and Their Structure in MILD Combustion

“…In practice, MILD combustion is achieved typically by injecting fuel and air into either a recirculating flow or a stream containing hot products. In these configurations, the conditions for MILD combustion are achieved despite the limited time available for the fuel to mix with oxidizer because of the high momentum of the jets (Awosope et al, 2006;Cavaliere and de Joannon, 2004;Dally et al, 2002Dally et al, , 2004Galleti et al, 2007;Katsuki and Hasegawa, 1998;Li and Mi, 2011). Also, there is a possibility to find inhomogeneous mixture containing pockets of unburned and burned gases.…”

“…The combustion efficiency is enhanced under MILD conditions due to high preheating temperature using recovered exhaust heat (Cavaliere and de Joannon, 2004;Katsuki and Hasegawa, 1998;Wünning and Wünning, 1997). The thermal NO x formation is suppressed significantly due to low combustion temperature rise and intense dilution (Cavaliere and de Joannon, 2004;Katsuki and Hasegawa, 1998;Krishnamurthy et al, 2009;Li and Mi, 2011;Mohamed et al, 2009;Wünning and Wünning, 1997). Typically, it takes few seconds to produce a substantial amount of thermal NO x at around 1900 K and this reduces to a few milliseconds when the temperature is about 2300 K (Wünning and Wünning, 1997).…”

Comprehensive numerical study of the Adelaide Jet in Hot-Coflow burner by means of RANS and detailed chemistry

“…According to Li et al [12], high temperature pre-heating of combustion air and the high-speed injection of fuel are the main requirements to achieve MILD combustion condition. Based on these requirements, model flames like the Adelaide Jet in Hot Co-flow (JHC) burner [13] and Delft JHC burner [14,15] were built to emulate MILD condition.…”