No.4 Reformer Heater NOx Reduction
The No. 4 Platformer Heater, located at a refinery in South Texas, is a multi-cell Arbor coil heater consisting of a radiant section, a convection section and a stack. This heater is utilized to heat the feed entering the Platformer reactors. The platforming reaction is an endothermic reaction and occurs in four stages. The feed outlet of each stage reactor is reheated in the Platformer heater.
The firing in these cells was accomplished by 36 numbers (before revamp) of natural draft burners located at the end walls on both the sides. These burners were originally designed for oil and gas combination firing. Refinery wanted to reduce the NOx emissions to 0.035 lb/ MMBtu (HHV) and address the high excess air requirement of the heater. FIS was approached to resolve this issue.
FIS revamp scheme consisted of following:
Existing 36 numbers of natural draft burners were replaced with 44 Nos. of new Ultra Low NOx burners, to achieve lower NOx emissions at the heater stack. The NOx emissions were truncated to 0.035 lb/MMBtu. The number of burners were increased to reduce the heat release per burner and thereby reduce the flame lengths.
FIS noticed that the heater was poorly controlled. A very high draft at arch caused very high excess air to enter the heater. All three stacks were provided with cable and winch type manually operated damper to adjust the draft at arch. FIS provided opposed blade type pneumatically operated damper which are easily monitored and accessed. FIS designed the heater with required excess air as 15% in cell A/B and 30% in cell C/D.
The stack dampers were provided to ensure that draft at arch level was maintained at the design value of (-0.1)” WC during all operating conditions.
The flue gases leaving the convection section enter the three individual stacks located above each convection cells. The existing 3 Nos. of stacks were retained with slight modifications. Lower module of the existing stack was modified to incorporate the new stack dampers and the upper module was amended to incorporate new reduced diameter stack tips. (conical shaped). This ensured that the flue gases were dispersed at a higher velocity than the existing.
The following table depicts the summary of the burner details: