Natural Gas-Based Ammonia Synthesis

Operation of natural gas-based plants follows a basic sequence whose details differ among various proprietary processes (fig. 6.7) Natural gas must first be purified to remove H2S and any dust or liquid particles. Initial filtering is followed by compression to reformer pressure and preheating, and H2S sulfur is removed by absorption by zinc oxide (ZnO). The gas is then mixed with superheated steam and reformed in two stages. Primary reforming, producing a mixture of H2 and CO, is done inside alloyed stainless steel tubes packed with a nickel catalyst in most ammonia plants these tubes are placed in a furnace heated by gas (or liquid fuel) to 750-850 °C. 

The first thermal reforming units operated at an only slightly elevated pressure. Subsequently increases of pressure, heat fluxes, and temperatures, as well as better 

Principal stages of natural gas-based ammonia synthesis. 

Tubes are made either of chromium-nickel or chromium-niobium alloys, and they are expected to last at least for 100,000 hours (more than eleven years). The latest primary reformer designs have dispensed with the furnace altogether, and the process is heated by gases from the secondary reformer. The reforming reaction is always the same 

The exit gas is oxidized in the secondary reformer, a cylindrical vessel lined with a refractory material whose lower part is filled with the catalyst the exothermic reaction results in temperatures on the order of 1,000 °C. Oxidation is necessary in order to reduce the amount of unconverted methane (anywhere between 5-15% of the gas is still unreacted at that point) and to isolate the appropriate amount of N2 for the synthesis gas containing the stoichiometric H2 N2 ratio of 3 1  

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