Multiscale analysis of combustion in novel CH₄/syngas-fueled supercritical composite working fluid cycles

The dual-CO₂/H₂O-reflux supercritical composite working fluid cycles (SCWFCs) can extend the operating flexibility of oxy-fuel power systems, but feasible cycle points must also satisfy flame-stability and combustor-burnout constraints. A multiscale evaluation was performed for CH₄ and representative CO/H₂ syngas fuels by coupling steady-state cycle analysis, one-dimensional flame calculations, and Chemical Reactor Network (CRN) combustor assessment. Flammability limits, ignition delay time, and laminar flame speed were examined at the fundamental combustion-characteristic scale, while fuel burnout was evaluated under representative combustor organizations and flow-distribution strategies. Dual CO₂/H₂O reflux widened the admissible operating space relative to single-species reflux, although the surviving windows depended strongly on fuel composition and diluent identity. The SCWFC mechanism, which integrates supercritical water (SCW) and supercritical carbon dioxide (sCO₂) elementary chemistry, gave the most consistent behavior from subcritical to supercritical conditions among the tested mechanisms. Under supercritical conditions, finite-rate combustion trends could not be directly transferred among fuel/diluent pairs. CO mitigation was governed first by boundary-condition adjustment and then by multi-stage pressure combustor integration. Combustion organization and reflux-zone design produced stronger CO suppression for CH₄ than for syngas. These results provide a constrained cross-scale basis for selecting SCWFC operating windows and guiding combustor design.