Research Of High Temperature Air Combustion Model And Optimization Of Burner Configurations

High Temperature Air Combustion (HTAC) is an advanced and efficient regenerative combustion technology, which has the advantage of both energy saving and extremely low NOx emission. HTAC was considered as one of the promising combustion technology in the 21st century and has been widely used in the industry fields of steel metallurgy, glass, ceram, cement, etc. Compared with the traditional combustion process, HTAC undergoes highly preheated air temperature and lean oxygen conditions. The combustion performance of HTAC is different from that of genearl combustin processes. Therefore reasonable combusiton models for HTAC are critical for nureical simulation. Three models of gas combusiton, i.e., eddy breakup model (EBU), EBU-Arrhenius model and Probability Density Function model (PDF) were applied to simualate the HTAC of a furnace of 2m×2m×6.25m and then a comparison of the numeical and expperimental results were made to determine the suitable model. The models for nitrogen oxide (NO) formation and conversion including thernal NO, prompt NO and the intermediate route of N2O as well as the NO reduction by reburning were taken into accout for NO emission simulation and the importance of the above NO models was compared. Based on these results, the final CFD models including the EBU combustion model, thermal and prompt NO formation model and NO reduction model by reburning were adopted for the HTAC simulation with different burners combined with the RSM turbulent model, the Discrete Odinate (DO) rediation model of gases and a WSGGM model of gas absorption factor. The effects of several operation and burner configuratin parameters on the HTAC performace were investigaed. The following conlusions can be draws from the results.(1) The results showed that the temperature field, concentration field fuel coponent, oxygen and NO predicted by EBU model agreed with the experimental results best. But the peak temperature position predicted by EBU model does not agree exactly with the measured data. Therefore, it is necessary to modify the related parameters of EBU model according to the nature of HTAC in order to optimize the predicted results. Related study results showed that the reaction speed of HTAC is slower than that of the traditional combustion. The empirical constant, A, in the EBU model was set a smaller value to decrease the combustion speed and the revised EBU model was used to carry out the simualtion. Results showed that the predicated temperature field, concentration field and the NOx formation process were greatly improved when A take the value of unity.(2) Results showed that Thermal NO was the main mechanism of NO formation, which took approximately the amount of 90% in the total NO formation. Prompt NO is the second largest NO formation process in HTAC, which was about 10% of the total NO formation. N2O intermediate mechanism was of little importan(?) which could be ignored in NO formation process in HTAC. NO reburning mechanism was an essential pass of NO formation in HTAC, without which the predicted results will be seriously higher than the measured.(3) Swirling burner could enhance the recirculation of the flue gas to enlarge the low oxygen combustion area, resulting in a more uniform temperature distribution in the furnace. Results also showed that the size and the position of high temperature and oxygen-rich region were the key reason of NO formation. Swirling burner can efficiently reduce the high temperature and oxygen-rich region and suppress NO formation.(4) Three technical approaches could be used to decrease NO emission, i.e., swirlig combustion, low inlet oxygen concentration and high inlet air velocity. When a swirling burener was used, the high temperature and oxygen-rich region decreased continuously with the increase of the swirling angle of the burner and NO emission decreased first to a minimim value and then increased again. Higher inlet combustion air velocity can suppress NO emission, but the NO reduction became small when the inlet air velocity exceeded 45 m/s. The NO emission was suppressed when lower inlet oxygen concentration combustion air was used, but the CO emission increased. Therefore, swirling burner was a better method for NO control not only because of its low NO emission performace, but also due to the fact that it's easy to combine with the other two approaches for a swirling burner to further improve the HTAV performance with an extremely low NO emission.(5) Different operation and configuration parameters obviously influenced the HTAC performance. Numerical results showed that the peak temperature and the mean temperature in furnace increased linearly with the increase of the preheated combustion air temperature. Temperature uniformity was improved, but the NO emission increased greatly. With the increase of the excess air ratio, NO emission first increased, and then decreased. When the swirling burner was employed, the NO emission was extremely low and the burnout was excellent even when the excess air ratio was set the value of unity. When the excess air ratio was large, the number of air passage has little influence on HTAC process, when the excess air ratio is small, the turbulence intensity in the furnace increased when the air passage increased and the burnout was improved. The stretch length of the swirling burner had important influence on the turbulence flow, combustion process and the NO formation in the furnace. With the increase of the stretch length, the high temperature and oxygen-rich region decreased continuously, NO emission were decreased first and then increased, which implied an optimal stretch length should be considered.