In the natural gas or naphtha based steam reforming, the use of excess air in the secondary reformer shifts some of the reforming load from the primary to the secondary reformer. Reducing the primary reforming is done by decreasing the heat supply.This in turn, increases the CH4 slip in process gas from primary reformer. It is shown that lowering of the primary reformer operating temperature by 20 °C results in 2% increase in unconverted methane content in the outlet stream. By increasing the internal firing by supplying more process air (up to 50% higher than in conventional reforming) in the secondary reformer, the same degree of reforming is achieved as in conventional reforming. The lower operating temperature reduces the energy consumption of the primary reformer, while at the same time increasing the lifetime and efficiency of the catalysts and reformer tubes (Ravi et al., 1989; UNEP, 1998 p.13). This measure is applicable for new plants (IPTS/EC, 2007 p.61) and allows for reducing the size and cost of the primary reformer.
Examples of processes that employ reduced primary reforming in combination with process air surplus in the secondary reformer are the Braun Purifier, the ICI AMV, the Foster Wheeler AM2, the Humphreys and Glasgow BYAS, the Montedison Low-pressure, the Kellogg’s LEAP processes and the Jacobs Plus Ammonia Technology. In some of these processes (such as the Foster Wheeler M2 and the Humphreys & Glasgow BYAS) a percentage of the desulfurized feed-gas overpasses the primary reformer and enters directly to the secondary reformer. Increasing flow of air in secondary reformer results in excess nitrogen, which has to be removed in gas purification section.In some cases, this removal is performed with the use of cryogenics after the methanation process (Ullman’s, 2011 p.238-240) which also removes lower level of methane impurities, giving advantage to the ammonia synthesis section (FAI, 2013). However, this measure increases both energy consumption (IPTS/EC, 2007 p.61) and the size of the process air compressor marginally (FAI, 2013). In addition, higher temperature of exhaust gases from secondary reformer require a more robust design of reformed gas waste heat boiler, located downstream. Also, cryogenic purification requires a separate air separation plant. All of these increase the capital costs.