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Chapter 22.4.2 Processes with reduced primary reforming and increasedprocess airDescriptionSome processes are designed for reduced primary reforming by transferring some of the duty tothe secondary reformer because of the marginal low efficiency of the primary reformer. Table2.10 shows the characteristic modifications in comparison with the conventional process.Process stepDecreased firing inthe primaryreformerIncreased processair supply to thesecondaryreformerShift conversion,CO 2 removal andmethanationCryogenic finalpurificationAmmoniasynthesisDescriptionIn this configuration, the transfer of some of the primary reformer function tothe secondary reformer reduces the level of primary reforming that is carriedout. The consequence of this is less firing and a correspondingly lower NO xformation. The heat supply in the primary reformer is reduced and the processoutlet temperature is lowered to approximately 700 °C, the firing efficiency isincreased, and the size and cost of the primary reformer are reduced. The milderoperation conditions prolong the life of catalyst tubes and the outlet header. Theextent of reforming is reduced according to the lower heat supply and lowertemperature. There is a slight decrease in the steam to carbon ratio, compared tothe conventional concept.A decreased heat supply in the primary reformer means that increased internalfiring is necessary to achieve approximately the same degree of total reforming.The slightly higher methane slip in this type of process is acceptable becausemost of the methane will be removed in a cryogenic purification step.The process air requirement is up to 50 % higher than in the conventionalprocess. This requires an increased compression capacity and energyconsumption. The process air compressor can be driven by a gas turbine, withthe exhaust gas from the turbine being used as combustion air in the primaryreformer. Some excess steam is available for export when a gas turbine is used.These steps are not significantly different to the conventional process. See Table2.6.In the purifier process, excess nitrogen, most of the residual methane and part ofthe argon are removed from the synthesis gas by condensation at a temperatureof around -180 °C. The separated methane and nitrogen mixture is used as fuelin the primary reformer. The purified synthesis gas is then almost free ofimpurities, except for a small amount of argon, helium and methane. Comparedto the conventional process, the high degree of purity avoids a sizeable purgegas stream in the ammonia synthesis loop. The flash gas which originates fromdepressurising the condensed ammonia, does entrain a small amount of argonstill in the loop. The cooling energy is provided by expansion of the main gasstream in a turbo expander and by the expansion of the waste gas fractioncontaining methane.The removal of essentially all the impurities from the make-up synthesis gas is asignificant improvement, compared to the conventional purification bymethanation only. Together, the higher conversion per pass and reduced purgeflow result in a more efficient ammonia synthesis loop.Table 2.10: Characteristics of processes with reduced primary reforming60 Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers