Methane

Methane, as the primary constituent of natural gas and biogas, is a critical feedstock for high-efficiency distributed power generation via Solid Oxide Fuel Cell–Micro Gas Turbine (SOFC–MGT) hybrids. However, under dynamic load conditions typical of grid-support applications—such as duck-curve ramps and ±30% step transients—these methane-fueled systems experience rapid thermal excursions and reformer instability, accelerating stack degradation and forfeiting significant efficiency. To address this, we present a co-design framework that systematically integrates thermal energy storage (phase change material, PCM) and electrochemical storage (battery) within the SOFC–MGT architecture. The SOFC model was calibrated at 1073 K, and the PCM and battery models were compared with published data (maximum deviations: SOFC 13%, PCM 2.2%, battery 6.3%); the reformer and MGT models were not validated. A heuristic Simple Additive Weighting (SAW) matrix is first used only as a qualitative screening step to confirm that all four candidate topologies are structurally feasible; no topology is eliminated or selected at this stage. Battery-Dominant obtains the highest SAW score (200) because SAW is fully compensatory: its high performance, dynamic stability and load-following ratings offset its weaker thermal buffering. All four topologies are then ranked with AHP and TOPSIS using metrics derived from 168 h transient simulations. Both methods, which share the same criterion weights and decision matrix, place the PCM-Integrated configuration first (AHP-weighted score 7.95; TOPSIS C_i = 0.790), with priority weights of 40.7% for efficiency and 31.0% for thermal stability. Battery-Dominant ranks second in both AHP (6.44) and TOPSIS (C_i = 0.384) because of double-conversion losses. A weighted-sum genetic algorithm then yields 75.0% peak and 68.2% average net electrical efficiency, defined as net AC output divided by the methane LHV input, including inverter (95%), battery round-trip and MGT parasitic losses. Under the same 168 h transient profile, this is 1.45 percentage points above the conventional series SOFC–MGT topology (66.75%). It is 12.2–13.2 percentage points above the 55–56% reported for conventional SOFC–MGT hybrids in steady-state studies. In the model, the PCM reduces the standard deviation of stack temperature by 46% relative to the no-PCM case (for the assumed heat-transfer coefficient), and the battery supplies 82% of the load-power deviations resolved at 1 min resolution. These results are model-based. The main limitations, including the SOFC voltage correlation, extrapolation below the 1073 K validation temperature, and the unvalidated reformer kinetics, are discussed.

Natural gas is increasingly used in stationary spark-ignition engines because of its abundance, high octane rating, and lower carbon dioxide emissions than conventional liquid fuels. Blending hydrogen into natural gas pipelines enables large-scale transport, but pipeline compressor engines must run directly on this blended gas. Many prior hydrogen blending studies on such engines have predominantly relied on idealized indoor testing platforms coupled with dynamometer loading to isolate the impact of hydrogen addition; however, this study experimentally evaluated hydrogen blends on a six-cylinder, four-stroke industrially representative engine-compressor unit loaded by a closed natural gas compression loop operating under open-air ambient conditions to analyze the system-level response to hydrogen addition. Additionally, emissions were characterized by using a high-resolution FTIR MKS 2030 MultiGas analyzer. Natural gas–hydrogen blends containing 0–30% hydrogen by volume were tested in 5% increments. Natural gas flow generally decreased with hydrogen addition, while brake thermal efficiency increased at all operating conditions. Relative to the natural gas baseline, methane emissions decreased by as much as 47%, volatile organic compounds by 62%, and carbon monoxide by 94%; however, NOx emissions increased by as much as 324%. Carbon dioxide emissions increased by up to 12% at lower loads and decreased by up to 13% at higher loads. At 30% hydrogen, the equivalence ratio decreased by 6.25% at the lowest tested load and 8.28% at the highest tested load. Hydrogen blending can, therefore, improve efficiency and reduce carbon-based emissions in industrial, rich-burn engines, but the accompanying NOx increase and leaner operation warrant further testing with adjusted spark timing and controlled equivalence ratio.

Atmospheric methane (CH₄) growth reflects both emissions and chemical removal, but their contributions remain difficult to separate because the dominant sink, reaction with hydroxyl radicals (OH), is incompletely constrained. This review links methane sources and attribution with OH variability, methane lifetime, atmospheric oxidation capacity (AOC), and climate. Tropospheric OH accounts for roughly 90% of methane removal. Methane oxidation consumes OH, contributing to a nonlinear feedback that extends its perturbation lifetime. Fire co-emissions, low-NO_x biogenic chemistry, reactive halogens, and climate variability add further spatial and temporal variability. Comparisons of methyl chloroform and hydrofluorocarbon inversions with chemistry models show that inferred OH trends and spatial patterns depend on tracer emissions, transport, and chemical mechanisms. The Amazon and boreal–Arctic regions show contrasting examples of coupled source and oxidant variability. The 2020–2023 methane surge further illustrates the difficulty of distinguishing enhanced wetland emissions from changes in OH-mediated removal. Future methane concentrations and the benefits of mitigation will depend on coordinated changes in methane and reactive precursor emissions. Better constraints on AOC, particularly OH variability and methane–oxidant feedbacks, would improve methane attribution and future projections.