Search

In order to improve the energy efficiency of hydrogen storage for power generation, in this work, an improved Graz cycle (R-Graz cycle) was proposed, which can realize more efficient hydrogen to electricity conversion. The modeling and optimization program of the cycle was developed based on MATLAB, and the thermodynamic properties of the working fluid were obtained by REFPROP. The results show that the R-Graz cycle can reach a net efficiency of 70.98% at the design point, which is about 0.5% higher than that of the Graz cycle with the same input parameters, and much higher than that of the modern gas-steam combined cycle. With the same amount of hydrogen input, the R-Graz cycle can produce 0.34% more electricity than the Graz cycle. At the design point, the levelized cost of electricity (LCOE) of the R-Graz cycle is 156.23 $/MWh, which is 0.22% lower than that of the Graz cycle, and the net present value is also higher. Analysis on the influence of key parameters shows that increasing the inlet temperature and pressure of the high-temperature gas turbine and the inlet temperature of the high-pressure steam turbine as well as reducing the condenser pressure are conducive to improving the R-Graz cycle efficiency, among which the influence of the inlet pressure of the high-temperature gas turbine and condenser pressure is particularly significant. This study also presents the impact of these key parameters on the LCOE. This research can effectively increase the conversion efficiency of electricity-hydrogen-electricity mode, and provides a new path for hydrogen utilization. • A revised Graz cycle for power generation based on hydrogen storage is proposed. • The net efficiency of proposed cycle is higher than 70%. • Total efficiency of about 50% can be achieved in the hydrogen storage scenario. • The economy of revised cycle is better than that of Graz cycle on LCOE and NPV. [ABSTRACT FROM AUTHOR]

The structure and thermoacoustic instability (TI) of premixed CH 4 /H 2 /air swirling flames were experimentally investigated for a range of hydrogen fraction (η H2) up to 80% under different equivalence ratio (Φ) and swirl number (S) conditions. It is shown that the onset of TI is enhanced when increasing either η H2 , S , or Φ. The dominant frequency of TI increases dramatically with η H2. The higher dominant frequency in the hydrogen-enriched flames can be attributed to a shorter flame length which results in a reduced flame convection time. It is observed that the unstable flames are always accompanied by the appearance of outer recirculation zone (ORZ) flame. Therefore, the flame kernel residing in the ORZ can be an indicator of the occurrence of TI. The flame front of thermoacoustic unstable flames was observed to be more wrinkled, e.g., with larger mean absolute curvature (κ abs) and local flame surface area ratio (δ Σ max). Importantly, the phase-locked analysis shows that κ abs and δ Σ max can be modified at different oscillation phases, and their maximum and minimum values are simultaneously achieved at phase angles θ of about 0° and 180°, respectively. Variations of κ abs and δ Σ max are in phase with the heat release rate, indicating a strong correlation between the TI and flame structure modification; however, they show a phase lag of about 72° behind the pressure in this work. These results are vital when understanding and predicting the TI based on the flame structure, especially when adopting a visual detection method of the instability. [Display omitted] • Thermoacoustic instability of swirling lean premixed CH 4 /H 2 /air flames is studied. • The dominant frequency increases significantly with hydrogen fraction. • Appearance of the ORZ flame is highly related to the instability. • The flame front is more wrinkled in unstable flames. • Flame curvature and surface area oscillate in phase with heat release rate. [ABSTRACT FROM AUTHOR]

This paper investigated methane/air flame characteristics with hydrogen addition in micro confined combustion space experimentally and computationally. The focus is on the effect of hydrogen addition on the methane/air flame stabilization, the onset of flame with repetitive extinction and ignition (FREI), and the global flame quenching in decreasing continuously combustion space. Furthermore, the effects of hydrogen addition on the flame temperature and the local equivalence ratio distribution were analyzed systematically using numerical simulations. In addition, the effects of hydrogen addition on the concentrations of OH and H radicals, and the critical scalar dissipation rate of local flame extinction were discussed. With a higher hydrogen ratio, the mixing is faster, and the flame is smaller. When the micro confined space is narrower, the heat loss to the combustor walls has a higher impact on the flames. The flames with higher hydrogen ratios have therefore lower peak flame temperatures and lower concentrations of H and OH radicals. The results show that hydrogen addition can effectively widen the stable combustion range of methane/air flames in the micro confined space by about 20% when the hydrogen addition ratio reaches 50%. The frequency and the maximum propagation velocity of FREI flames can be increased as well. The quenching distance of methane/hydrogen/air flames decreases nearly linearly with the increase of hydrogen ratio. This is attributed to the higher critical scalar dissipation rate of local flame extinction in flames with a higher hydrogen ratio. • Flame characteristics of CH 4 /Air with H 2 addition in the micro space were studied. • Adding H 2 can broaden the flame stabilization zone and reduce the quenching distance. • H 2 enhanced flame stabilization is due to the increased SDR of local flame extinction. • Hydrogen addition leads to a faster mixing of the fuel/air mixture. [ABSTRACT FROM AUTHOR]