Auto-Thermal (Secondary) Reforming

Auto-Thermal (Secondary) Reforming Secondary Reforming or as sometimes called, "Auto-Thermal Reforming" is included in plants as an extension of Primary Reforming to lower the Methane content of synthesis gas from nominal levels of around 10 volume percent to about 0.2 volume percent.  (Dry basis) This is achieved by introducing process air into the Primary Reforming effluent synthesis gas stream in a combustion zone above (Secondary) Reforming catalyst.  The combustion (mixing) zone performance is highly critical to the overall Secondary Reforming catalyst performance, since little further mixing occurs within the catalyst bed itself.  Much effort has been spent by many vendors in optimal design of "Air-Mixers" to introduce process air into Secondary Reformers.  The most common designs have several similarities, insuring good mixing performance, permitting a uniform adiabatic "flame" temperature above the Reforming catalyst bed.  Multiple nozzles are employed, with the number and size depending closely on the air velocity at the nozzle tip to produce the required kinetic energy for good fluid mixing, commonly as follows.
	Number of Air Mixer          Typical Sizing Air Nozzles                            Velocity At Nozzle Tip 12-20                               250-275 Feet/Sec 25-40                               100-200 60 or more                        40-75 12-20                               76.2-83.8 m/sec 25-40                               30.5-61.0 60 or more                       2.2-22.9
	The more complicated the air-mixer design, the greater the cost.  One vendor has been particularly successful with high performance air mixer designs requiring only 12-20 nozzles.  The process air sizing pressure drop for air-mixers using a 12 nozzle arrangement is 5-6 psi, (0.35-0.42 kg/cm2), corresponding to 275 Feet/Sec (83.8 m/Sec) tip velocity.  With poor process air mixing, localized gas temperatures can exceed 2700 Deg F (1482 Deg Centigrade) at the inlet of the Reforming catalyst. Generally, protection of the leading edge layer of Reforming catalyst from direct flame exposure which otherwise results in sintering and shrinkage is accomplished by including a layer of high refractory material on top of the catalyst, such as Alumina "Target Brick" or irregular lumps.  Target Brick can be designed to promote gas distribution, generally with a mixed gas pressure drop of about 0.1-0.2 psi (0.007-0.014 kg/cm2). Some Reforming may occur in the mixing zone, before entering Reforming catalyst, which would tend to reduce the theoretical flame temperature, but this is generally ignored.  Depending on Primary Reforming operating temperature, process air preheating and process air rate, theoretical flame temperature normally operates up to 2300 Deg F (1260 Deg Centigrade) inlet the Reforming catalyst.  Consequently, Secondary Reforming catalyst must be designed to continuously operate in such sever temperature conditions without thermal attrition or deformation.  This requires a highly refractory catalyst support material, but the amount of active Nickel on the catalyst can be reduced to about 60-80 percent of the requirement for Primary Reforming catalysts, while still promoting adequate Reforming activity.  Reforming catalyst strength is foremost in the Secondary Reforming application to promote long life, which is commonly 5 or more years.