Abstract
In the past 25years high temperature air combustion (HiTAC) technology has been proved and utilized in industry as a promising way to increase thermal efficiency, create a relatively uniform temperature distribution, and reduce emissions of harmful pollutants such as NOX and CO. However, due to the complexity of fuel-oil combustion, to date HiTAC is mainly applied to gaseous fuel or coal, and little is known about spray combustion under HiTAC condition. In the present study, we numerically investigate the Delft Spray-in-Hot-Coflow (DSHC) using ethanol in high temperature diluted combustion air, and extend it to more co-flow conditions. We employ different temperatures and oxygen concentrations of the co-flow in order to dilute the oxidizer/fuel before it reacts with the fuel/ oxidizer. The pressure-swirl atomizer model with an Eulerian-Lagrangian approach was implemented for the spray modeling. Collision, coalescence, secondary breakup and evaporation of the drops were taken into account. The steady laminar flamelet model for the combustion of ethanol, the Discrete Ordinate model for radiation and the k-ε model for the turbulence with enhanced wall treatment were validated by the simulation of the NIST flame under conventional conditions and then used in the current study.The results indicate that the decreased peak temperature in many HiTAC applications with high temperature combustion air is mainly due to the reduced oxygen concentration by entraining flue gas.In the present study, a low oxygen concentration slows the evaporation process of droplets. It results in an enlarged combustion zone, a lowered peak temperature and minor NOX emission. However, decreasing the oxygen concentration may lead to problems of cracking, soot formation and flame extinction, especially for heavy oils. The optimization needs to be carried out based on the analysis of a specific fuel in order to create a HiTAC-like condition.Based on the results of the current study, the 1500K and 6%vol oxygen concentration case is considered as a HiTAC condition.
Highlights
High temperature air combustion (HTAC) is a promising technology for energy saving, flame stability enhancement and reduction of NOX emission
Due to the complexity of fuel-oil combustion, to date high temperature air combustion (HiTAC) is mainly applied to gaseous fuel or coal, and little is known about spray combustion under HiTAC condition
An ethanol spray is produced by a pressure swirl atomizer, whereas the hot co-flow is assured by secondary burner whereby air and Dutch natural gas (DNG) mix and generate a matrix of 236 lean flames
Summary
High temperature air combustion (HTAC) is a promising technology for energy saving, flame stability enhancement and reduction of NOX emission It has been applied in many experimental and industrial applications, and developed and reported as “moderate or intense low oxygen dilution (mild) combustion”, “flameless oxidation”, or “colorless distributed combustion (CDC)” [1,2,3,4,5,6,7,8]. To address all the relevant phenomena, we employ a combination of models, such as models for turbulence, atomization, secondary break-up, collision and coalescence, evaporation, radiative heat transfer, combustion, etc These models have been validated already by the simulation of the NIST flame using methanol under conventional condition [12] and were applied for comparison with preliminary results from the ethanol spray-in-hot-co-flow [13]. They represent spray flames under conventional condition and towards-HiTAC condition, respectively
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