Utilization of hydrogen-natural gas mixtures or pure hydrogen in gas-fired power plants leads to a lower carbon footprint of these power plants. Recently, reducing carbon emissions of gas power plants is gaining traction, as more coal-fired power plants are forced to retire. Increased interest in the hydrogen economy has further directed discussions to replace natural gas with hydrogen. Global hydrogen (H2) production is projected to boom in the coming years. During this period, most of the produced hydrogen will be employed for energy purposes.Power-to-gas (P2G) is a technology that uses low carbon electricity, such as excess electricity generated by renewable energies to produce a gas fuel, particularly green hydrogen. In P2G technologies, hydrogen is produced through electrolysis, and in some cases, the produced hydrogen is further converted to synthetic gas, liquified petroleum gas, or methane. The produced gas fuel is used in different sectors including heat supply, power generation, and chemical processes, thus expanding the penetration of clean energy into other energy sectors.The levelized cost of hydrogen is highly dependent on the production means: grey/brown hydrogen (using fossil fuels), blue hydrogen (using fossil fuels with carbon capture and storage), and green hydrogen ((using low carbon electricity). The final cost of green hydrogen is mainly influenced by the cost of electricity, the load factor, and the capital expenditures associated with the electrolyzer.The paper evaluates the impact of hydrogen mixtures on existing power plants in 3 different countries: Denmark, Germany, and the United Kingdom. The investigation is carried out using energetic, exergetic, and economic analyses to depict the implications of the various mixtures on each of the power plants. The simulation of the power plants was performed using the software Ebsilon Professional. For the natural gas average prices were used, whereas for hydrogen levelized costs of its generation in alkaline and polymer electrolyte membrane electrolytic units in the different countries were applied.Only four (4) important dependent variables were chosen for discussion: Levelized cost of fuel, exergetic efficiency, NOx, and CO2 emissions. The hydrogen blend percentages are nine, the first is dependent on the country’s highest natural gas blend presently or in the nearest future, while the remaining are 25vol%, 40vol%, 50vol%, 60vol%, 75vol%, 90vol%, 95vol%, and 100vol%. 90vol% and 95vol% were included because the H2 mass fraction goes past 0.5 only at 90vol%.The results show that hydrogen introduction into the gas fuel caused lower CO2 emissions, and higher energetic and exergetic efficiencies (both for the entire system and the combustion chamber) and automatically lowered the mass flow rate of fuel. However, NOx discharge, levelized fuel cost (with the exception of the Viborg power plant), and volumetric flowrate increased significantly due to mixed H2.The Viborg power plant (Denmark) demonstrated an anomaly, its levelized cost of fuel decreased due to soaring gas prices, which are comparable to the relatively low price of green hydrogen in Denmark and the lower mass flow rate of fuel due to increasing efficiency with increasing hydrogen content in the mix. The surging NOx emissions are mainly attributed to the thermal NOx mechanism. The mechanism is favored by the increasing combustion temperature. NOx formation is highly dependent on the temperature of the incoming air and highly dependent on the conditions of the combustion process (stoichiometric, lean, or rich conditions).In the future, an exergoeconomic analysis should be conducted to evaluate the effects of mixing green hydrogen in existing gas-fired power plants and to determine the optimal blend of hydrogen in existing power plants while considering important plant constraints.

Evaluation of “Natural Gas / Hydrogen” Mixtures for Power-to-Gas Application

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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