(72e) Burner N0X from Ethylene Cracking Furnaces

Allowable emission limits for nitrogen oxides (NOx) remain under scrutiny as regulatory agencies continue to assess the impact of NOx emissions on ambient air quality for ground-level ozone. The ozone is formed in the atmosphere by the sunlight-induced reaction of NOx with certain hydrocarbons. It is understandable that operators of affected combustion sources would prefer to achieve compliance with any such NOx reductions by means of burner modifications, rather than through more costly and complex post-combustions controls, such as selective catalytic reduction (SCR). For smooth project execution, accurate prediction of the extent of burner-NOx reduction is critical. However, computational fluid dynamics (CFD), used to model flame patterns, furnace temperatures, and the like, has done a poor job in predicting burner NOx. Likewise, burner testing in a manufacturer's pilot facility often produces low estimates for NOx when compared to a full-scale furnace. A viable alternative is an empirical approach based on kinetic theory and validated by numerous field data. This paper shows the results from a new correlation of NOx emissions for ethylene cracking furnaces. It is derived from an established NOx correlation for commercial steam-methane reformer (SMR) furnaces, while recognizing the differences in fuels and furnace conditions between the two processes. It uses adiabatic flame temperature (AFT), excess furnace oxygen (O2), and furnace temperatures. Calculations can be accomplished rapidly and allow one to compute absolute values of NOx, explore changes in NOx from a base case, and explain experimental observations. Calculated values compare favorably with available NOx data reported for commercial ethylene furnaces spanning a wide range of conditions. The correlation can also be tailored to fit individual furnace data for even better agreement.