Scope 1 & 2 emissions CO capture rate
Natural gas & oxygen consumption
A
B
1.0
0.6
100
0.4
50
0.5
0.2
0.0
0
1
2 3
4
5
6
7
8
0.0
Case number
1
2
3
4
5
6
7
8
Case number
Scope 1 emissions Scope (1+2) emissions
Scope 2 emissions CO capture rate (%)
(Feed + fuel) NG consumption
Oxygen consumption
Figure 5A Scope 1 and 2 emissions, CO 2 capture rate 5B Natural gas and O 2 consumption
turbines for power generation, with net electricity credited as plant export. All other OSBL equipment, including the ASU, used motor drives, while the high-pressure boiler feedwater pump operated on steam or on motor as required. Process steam for the water-gas shift reactors was supplied from the process condensate stripper and the medium-pressure steam header. A minimum steam-to-gas ratio of 0.5 was maintained at the shift reactor inlet, with higher ratios applied as needed to control shift outlet temperature within catalyst and metallurgical limits. Emissions and carbon intensity evaluation CI calculations include Scope 1 direct emissions from combustion and process releases and Scope 2 emissions associated with imported grid electricity, using Louisiana regional grid factors. Pre-combustion CO₂ capture was set at ~99.9% using conventional acid-gas removal, while post-combustion capture was limited to 95%. The study boundary excludes upstream natural gas emissions; therefore, the results represent comparative plant-level emissions rather than full lifecycle CI. Figure 5A shows significant differences among the configurations. Case 2 reduces direct emissions through combined pre- and post- combustion capture but remains constrained by residual reformer flue-gas emissions associated with the assumed 95% post-combustion capture level. Cases 4 through 6 achieve capture rates
exceeding 98% by concentrating most carbon in the syngas for high-efficiency pre-combustion removal, combined with carbon recycle, high hydrogen recovery, convective reforming where applicable, and hydrogen-rich firing to minimise direct heater emissions. However, the resulting oxygen and electrical requirements can increase Scope 2 emissions. Consequently, high CO₂ capture does not necessarily correspond to the lowest combined Scope 1 and 2 CI under the assumed grid power basis. POx-based Cases 7 and 8 also achieve high capture performance because of low methane slip, high-temperature conversion, and integrated heat recovery that minimises conventional direct-fired heating. The comparison demonstrates that several process architectures can achieve deep direct-emissions reduction, while overall CI depends strongly on steam and power integration and the CI of imported electricity. Recycle reduction for lower carbon intensity Lower CI can be achieved by minimising the recycle of carbon-containing streams to the reforming section, which requires reducing residual CH₄ and CO in the syngas/ H₂ stream. Methane slip is primarily controlled by the reforming conditions selected for each configuration, leaving limited scope for further reduction. Residual CO, however, can be reduced to single-digit ppm levels using either conventional methanation or selective catalytic oxidation.
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