Steam methane reforming

Steam methane reforming, or SMR, denotes the most widespread hydrogen production route in the nitrogen fertiliser industry. This process converts light hydrocarbons, chiefly methane from natural gas, into a synthesis gas rich in hydrogen, which is then combined with atmospheric nitrogen in the ammonia synthesis loop to form NH₃. The overall theoretical reaction is written as: 0.88 CH₄ + 1.26 air + 1.24 H₂O → 0.88 CO₂ + N₂ + 3 H₂, then N₂ + 3 H₂ → 2 NH₃^[1]. Synthesis gas generation takes place at 25-35 bar, whilst ammonia synthesis operates between 100 and 250 bar^[1].

Description of the conventional process

The flowsheet comprises several stages: desulphurisation, primary reforming, secondary reforming, CO shift, decarbonation, methanation, compression and synthesis^[2].

Desulphurisation is critical because catalysts are sensitive to sulphur-bearing compounds. The gas, which may contain up to 5 mg S/Nm³, is preheated to 350-400 °C then treated: sulphur compounds are hydrogenated to H₂S over a cobalt-molybdenum catalyst, then adsorbed on pelleted zinc oxide^[1][2]. Sulphur is reduced to below 0.1 ppm^[1], with the hydrogen required coming from recycle in the synthesis section^[1][2].

The purified gas is mixed with process steam at a steam-to-carbon (S/C) ratio of roughly 3.0 for MTD processes^[1]. The mixture is raised to 500-600 °C before entering the primary reformer^[1], consisting of nickel-chrome alloy tubes with a nickel catalyst. The endothermic reaction requires heat input to reach 780-830 °C at the outlet^[1]. Only 30-40% of the feed is reformed at this stage^[3].

Secondary reforming raises the temperature through internal combustion with process air, which supplies the nitrogen for synthesis. The air is heated to roughly 600 °C and injected via a burner; the outlet reaches roughly 1,000 °C with 0.2-0.3% residual methane on a dry basis^[3]. Cooling takes place in a waste heat boiler at 330-380 °C^[4].

CO conversion (12-15% on a dry basis^[3][4]) proceeds in two stages: first over an iron-chrome catalyst at 350-380 °C^[4]. The reaction CO + H₂O ↔ CO₂ + H₂ releases 41 kJ/mol^[3][4].

Variants and optimisations

Reduced primary reforming shifts part of the thermal duty to the secondary stage^[5]. The primary outlet temperature is lowered to roughly 700 °C, extending equipment life^[5]. Air to the secondary stage increases by up to 50% compared with the conventional process^[6]. Cryogenic purification at roughly -180 °C removes excess nitrogen and residual methane^[6].

Installation of an adiabatic prereformer reduces the thermal load by 5-10%^[7].

Energy integration

Units have surplus heat recovered to raise high-pressure steam driving the turbocompressors^[5]. A modern plant can achieve energy self-sufficiency^[5]. Consumptions for advanced conventional processes range between 29.2 and 31.5 GJ (NCV)/t NH₃, against 32.7 GJ/t for standard processes^[8].

The CO₂ generated, 1.15 to 1.30 kg per kg of ammonia^[9], may be captured for urea^[10]. Ammonia, liquefied at -33 °C, contains 82% nitrogen^[10].