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The rising demand for natural gas (NG) and hydrogen, due to their lower carbon footprint and role in storing surplus renewable energy, has highlighted the focus on developing advanced storage technologies. Traditional methods like liquefaction and compression face high energy and safety challenges, prompting the exploration of new solutions. Among these, hydrate-based gas storage stands out for its environmental benefits, using clathrate hydrates to store gas with low energy consumption and carbon emissions. Furthermore, the composition of hydrates, predominantly water (∼85%), and their lack of by-products during repetitive storage–release cycles firmly establish them as environmentally friendly gas storage media. However, kinetic challenges such as stochastic nucleation, limitations in mass and heat transfer, and thermodynamic barriers arising from harsh hydrate formation conditions have hindered the practical application of hydrates. While mechanical methods to improve hydrate formation exist, their use significantly increases the demand for electrical energy. Therefore, developing methods for gas hydrate formation under static conditions is crucial for utilizing this material as a safe and green gas storage medium. This review examines theoretical studies and experimental efforts to enhance hydrate formation kinetics in static systems without additional mechanical methods. Thermodynamic hydrate promoters to increase the driving forces for hydrate formation under mild conditions, surface-modified materials to increase nucleation probabilities for shorter induction times, and porous materials to provide pathways for mass and heat transfer have been widely investigated. The discussion addresses the direction and necessary efforts for utilizing hydrate-based gas storage as a next-generation green technology. [ABSTRACT FROM AUTHOR]

• Effects of piston shape and multi-angle nozzle were evaluated for DF RCCI operation. • Enhanced air utilization w/multi-angle nozzle reduced combustion and HT losses. • The highest GIE of RCCI improved as 51.8 % by hardware modifications. • Intake air amount could be increased by lowering CR (from 17 to 15) at the maximum load. • The maximum load of RCCI could be extended from 81 (reference) to 101 kW of BP. In this study, the effects of modifications of both the piston shape and injector nozzle are experimentally investigated for high load extension of a natural gas–diesel dual-fuel reactivity-controlled compression ignition (RCCI) engine. First, a bathtub shape with a reduced compression ratio is applied to the modified piston which was reported to improve the flame propagation in squish volume, the cooling loss, and the air induction rate under the same limit of in-cylinder pressure from previous studies. In addition, a multiangle two-layer injector is used to extend the maximum load, which has been proven to be effective for unburned hydrocarbon emissions and gross indicated efficiency in part-load operation in our previous study. The experiments are conducted under both 1200 rpm part-load operation (approximately 1950 J/cylinder per cycle of fuel input energy) and 1800 rpm maximum load operation. The objectives are to comprehensively evaluate the effects of the modifications on natural gas–diesel RCCI operation and verify the effects of all load conditions. The experimental results reveal that the changes in the piston—reduction in the compression ratio and a bathtub piston shape—are effective for both NOx and smoke emissions as the diesel injection timing is advanced. Although the multi-hole double layer (combined narrow and wide injection) has less effect on emissions compared to that of the change in piston, it affects the mixing process inside the cylinder, thus retarding the combustion phasing, prolonging the combustion duration, and eventually reducing the cooling loss. Both modifications are highly effective for advanced diesel injection timing corresponding to RCCI operation, which extends high-load operation from 80.9 to 101 kW of brake power, relatively 24.8 % increase, by adopting both the piston and injector modifications. Based on the modifications, the mass fraction of natural gas is suggested to be approximately 80 %. A higher fraction causes the variation in the maximum in-cylinder pressure to be extremely sensitive to allow robust operation. In contrast, if it is lower, the amounts of both NOx and smoke emissions exceed the emission standard levels (0.4 g/kWh and 15 mg/kWh for NOx and smoke emissions, respectively, corresponding to Stage-V of non-road engine operation). [ABSTRACT FROM AUTHOR]

• Effects of multi-angle injector on DF combustion was evaluated. • Multi-angle injector helped to reduce NMHC emissions under RCCI. • Multi-angle injector showed better GIE due to lower HT loss. • GWI could be improved more by multi-angle injector under RCCI. In this study, natural gas and diesel dual-fuel combustion in a heavy-duty compression ignition engine is experimentally evaluated by adopting a modified multi-angle nozzle of a diesel injector to reduce the global warming impact (GWI) factor while maintaining high thermal efficiency. The original injector has a single layer with an umbrella angle of 150° and eight holes, whereas the modified injector has an additional layer below with an injection angle of 90° and four holes. The engine speed is 1,200 rpm under low load conditions (intake pressure of 0.11 MPa and overall equivalence ratio of 0.5, such that the torque is varied); meanwhile, the natural gas fraction is varied from 45% to 71% while considering the combustion stability. The diesel injection timing is varied from the top dead center to 70° before top dead center at intervals of 10° for the injector nozzle conditions. Results show that the modified injector improves combustion efficiency under the reactivity-controlled compression ignition (RCCI) regime. In addition, the GWI factor is reduced by 5.6% compared with that of the original injector. Hence, RCCI combustion by early diesel injection using a multi-angle nozzle affords low emissions while considering the GWI factor, as well as maintains a high gross indicated thermal efficiency of 48.6%. [ABSTRACT FROM AUTHOR]

• Effects of fuel fractions and diesel SOI on THC are discussed. • Methane is related with crevice effect and bulk quenching. • NMHC is related with wall-wetting of diesel spray. • MFB50 position has strong influence on heat transfer loss. • RCCI improved the BTE with reduction of THC emissions. In dual-fuel combustion, the levels of total hydrocarbon (THC) emissions have increased, unlike the neat diesel compression ignition (CI) combustion. Intuitively, it can be inferred that most THC emissions are produced by the crevice effect of low-reactivity fuels (high-volatility fuels) under dual-fuel combustion. Additionally, it is possible that high levels of THC emissions are produced by dual-fuel combustion. If reactivity-controlled compression ignition (RCCI) combustion is applied by advancing the diesel injection timing to improve the premixing conditions, the occurrence of the wall-wetting effect may induce the production of high levels of THC emissions owing to incomplete combustion. Thus, in this study, the characteristics of non-methane hydrocarbons (NMHC) and methane emissions under general dual-fuel combustion conditions (diesel injection timings before top dead center (BTDC) 0 and 30 crank angle degree (CAD)) and RCCI conditions (diesel injection timings at 60 and 84 CAD BTDC) were investigated under a low-load in a 6-L heavy-duty engine. The natural gas fraction varied from 50% to the maximum level considering the combustion stability (coefficient of variance of indicated mean effective pressure). The results revealed that the overall THC emissions from the general dual-fuel combustion were higher compared with those of RCCI combustion, owing to the methane emissions from the squish area and the lower average in-cylinder temperature. Notably, methane emissions were dominant in the THC emissions (52–87%) under the general dual-fuel combustion condition, while NMHC emissions were a major part of the in THC emissions (61–87%) under the RCCI regimes. Hence, combustion inefficiency is a major concern in dual-fuel combustion. In this study, this problem was resolved by advancing the diesel injection timings. The findings in this study can be useful in the design of oxidation catalysts, regardless of whether the combustion regime is a general dual-fuel combustion or RCCI combustion considering the portion of methane emissions in the exhaust gas. Additionally, it is known that the RCCI combustion is superior to the general dual-fuel combustion with respect to higher thermal efficiency and low harmful THC emissions. [ABSTRACT FROM AUTHOR]