Numerical Analysis of NO and CO in a Flameless Burner

Paper from the AFRC 2016 conference titled Numerical Analysis of NO and CO in a Flameless BurnerCombustion of fossil fuels provides up to 80% of the energy produced worldwide, leading to considerably large emissions of CO2 and NOx. Motivated by more stringent legislations the engineering and research community addresses its efforts towards cleaner and more efficient technologies. CO2 is considered as the primary cause of global warming whilst CO and NOx are harmful for the human health. Modern combustion technologies aim to reduce CO, CO2 and NOx emissions with more efficient and less pollutant designs. Emissions abatement systems are commonly divided into three main techniques: i) pre-combustion, ii) post-combustion, and iii) combustion modification. In pre-combustion techniques, cleaner fuels are used to reduce the fuel bound NOx. Oxy-combustion also belongs to this group, where oxygen is separated from the air prior to combustion. On the other hand, in post-combustion techniques exhaust gases are treated with pollutants reducing agents or filters to meet environmental emissions restrictions. Common designs for combustion modification involve cleaner combustion include low excess air techniques, air or fuel staging, over-fire air reburning and flameless combustion.; This work focuses on the third technique using flameless combustion (FC). FC has attracted much attention because it represents a more efficient technology due to its very low CO, NOx and soot emissions. Burnt gases are recirculated at a high rate, with pre-heated oxidizer to sustain combustion and also avoid quenching. In such systems the oxygen content is low, resulting in reduced and smoothed local temperature peaks. The benefit of such systems is twofold, the general efficiency of the process is increased and the thermal NOx are reduced due to the lower temperatures. Computational Fluid Dynamics (CFD) is a valuable tool to improve the design of flameless burners with low development costs. Two main aspects are critical for high quality numerical prediction of FC burners, namely the slow auto-ignition process due to the dilution of burnt gases and the turbulent mixing of the fuel/air/burnt gases.; In this paper, a flameless combustor burner is numerically examined and the results are scrutinised with the available experimental data. The experimental data are well characterised and cover a range of operating conditions. The data are therefore an excellent candidate for validating CFD models. The objective of this work is to compare the performance of two combustion models; the Flamelet Generated Manifold (FGM) model and the complex chemistry model. Numerical modelling is performed with the commercial software STAR-CCM+ v.11.02 and the tabulation of chemistry is performed with DARS v3.02. High fidelity calculations in the context of LES are performed to obtain a better representation of the turbulent mixing. The models are used to examine the formation of CO and NOx emissions. The advantages/disadvantages of the FGM and complex chemistry models will be discussed including the benefits of LES that is able to capture the important mixing scales critical in predicting the correct flame dynamics and emissions