sCO2 Allam-Fetvedt oxy-combustion power cycle

Natural gas burns in near-pure oxygen, diluted by a large recycled supercritical-CO2 flow instead of air's nitrogen — the oxy-combustion, near-critical-CO2 cycle that yields pipeline-ready CO2 with no separate capture step. Main compressor takes CO2 from just above its critical point (310 K, 8 MPa vs. Tc=304 K/Pc=7.38 MPa) to 30 MPa; the combined CO2+combustion-product stream expands through a real isentropic-efficiency turbine, still supercritical throughout. Honesty notes: (1) this is an open-cycle v1, not the fully closed recuperated loop — the compressor's CO2 feed and the turbine's exhaust aren't tied together via a recuperator/recompression recycle yet (a stated follow-up), so read this as 'does the near-critical compression + oxy-combustion + supercritical expansion chain converge and deliver net power', not a bounded plant design. (2) The combustor's ambient_temperature param is reused as the recycled-CO2 inlet temperature (a naming artifact of fired_heater.py's air-furnace origins), not true ambient. (3) Turbine-inlet temperature here (~760 K) is lower than a commercial Allam design's ~1150 degC — that gap is exactly what the missing recuperator would close by preheating the recycle CO2 before combustion.