CFD Air Flow Channel Optimization for a Low NOx Operation of a Big Steam Boiler

One of the Romanian big power plant (Turceni) of 2400 MWe installed power has seven Benson boilers of 1035 t/h each, using Romanian pulverized lignite. Until 2008, every one of them has to be revamped in order to emit less than 500 mg/Nm(3) of NOx at full load and 6% oxygen content in the flue gas. The problem has been very complex due to the fact that all these steam boilers have been commissioned since 1968, without any design preparation concerning low NOx emissions. Step by step, all the technological measures have been performed, so the burning process of the Romanian low heat power lignite, has been able to produce low NOx emissions by the help of supplementary stages of air injections. The last problem and the Subject of the presented paper, Occurred regarding the possibility to inject enough and equal airflow rate through sixteen nozzles before the end of the boiler's furnace. The complexity of this problem consisted not only in a lot of technical constraints concerning to the starting place of the air pipes, but also to the path of the pipes (with very big dimensions) and to a very low influence in pressure drop losses towards the existent air ducts. A special supplementary problem has been that one to create the simplest shape of the air inlet elbow from the main existent air ducts, in order to suck the strictly necessary airflow rate at the lowest possible pressure drop loss. Due to the fact that the boiler's dimensions are too big to use only one air pipe toward the nozzles, two air manifolds are considered starting from two main different air ducts and feeding eight nozzles each; this Fact has complicated the task further, in order to balance the airflow rate inside the two manifolds. This complex problem has been solved by the help of CFD Fluent code and has been considered also the Subject of one of the team member's PhD dissertation. Special meshing techniques have been used in order to optimize the cells' number. For CFD results validation a special 1:50 transparent scaled model of the furnace has been constructed and performed using a laser - Doppler anemometer to determine the airflow rate and velocity, inside the air channels and at the nozzles' exits to the furnace.