Hydrogen purification: PSA vs N₂ wash ATR and POx configurations with PSA differ from nitrogen wash primarily in the hydrogen separation method. PSA uses cyclic adsorption to remove impurities, while nitrogen wash relies on the integration of cryogenic separation into the synthesis loop. This difference influences energy consumption, process integration, and overall flowsheet optimisation. Single-stage PSA provides lower hydrogen recovery, increasing feed demand. However, its larger off-gas stream can support steam generation and reduce net power consumption. Two-stage PSA achieves hydrogen recovery comparable to nitrogen wash, although at the cost of additional compression duty. Nitrogen wash provides high hydrogen recovery and direct synthesis-loop integration. It suits large plants but requires refrigeration, is more capital-intensive, and offers less modular flexibility. The preferred option therefore depends on site-specific economics, integration opportunities, and operating constraints. Comparison of feedstock, fuel, and primary utility requirements provides further insight into the eight configurations. Figure 5B shows significant differences in natural gas and oxygen consumption. Case 2 uses moderate natural gas consumption with no oxygen requirement, although further reducing direct furnace emissions through hydrogen-rich firing would increase natural gas demand and require substantial heater modifications. ATR-based Cases 4 and 5 consume more natural gas and oxygen because they require recycle and increased syngas production to generate hydrogen-rich fuel. Case 6 behaves differently: the convective reformer recovers high-temperature heat to provide additional reforming duty, reducing feedstock and oxygen consumption but also lowering high-pressure steam generation. Consequently, the steam system cannot support the large syngas and ammonia compressors, so motor drives are required. POx-based Cases 7 and 8 achieve very low direct emissions through simplified heat integration: higher oxygen consumption enables operation without conventional direct-fired process heaters while maintaining very low methane slip and high CO₂ capture performance.
Residual CO removal Selective CO oxidation could reduce residual CO in shifted syngas to single-digit ppm levels. Lower residual CO can reduce the need to recycle carbon-containing off-gas to the front end (ATR or POx), thereby lowering carbon intensity and potentially reducing both capital and operating costs. Selective CO oxidation can remove CO with substantially lower hydrogen loss than conventional methanation, provided high selectivity is maintained. This can improve hydrogen utilisation, synthesis-loop efficiency, and overall carbon capture performance. Key challenges include sensitivity to gas impurities, limited large-scale commercial experience, and the cost of precious metal catalysts. The technology therefore offers potential benefits for deep-decarbonisation ammonia projects. However, it requires careful evaluation of long-term catalyst performance, reliability, and economics. Methanation becomes less attractive for deep decarbonisation schemes because it consumes hydrogen and converts residual CO to methane. The resulting methane increases inert loading and purge losses and can ultimately raise carbon intensity if the carbon is released rather than captured. Techno-economic viability The economic comparison highlights important differences in capital cost, operating cost, and long-term project value across the eight cases. Figure 6 shows that Case 2, using SMR with SR and combined pre-and post- combustion capture, has the lowest capital and operating cost indices among the configurations evaluated. However, this advantage must be weighed against the scale and complexity of the post-combustion capture system. At 3,500 tpd capacity, the flue gas capture system becomes very large, increasing solvent circulation, equipment size, auxiliary power demand, and operating complexity. Case 2 also remains constrained by the assumed 95% post- combustion capture efficiency shown in Figure 5A, leaving residual furnace emissions. Hydrogen-rich firing could further reduce furnace emissions, but application to a large SMR heater would require substantial design
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