Numerical simulation of the combustion process of blends of hydrogen and natural gas for power generation

The tightening of international regulations in terms of climate-altering emissions is leading the energy production sector toward decarbonization and fuel flexibility for gas turbine technologies. Hydrogen, as a zero-carbon emission energy carrier, can play a primary role in the decarbonization process as it could also be exploited to store the excess of electricity produced by renewable energy sources whose availability and production rate are intrinsically fluctuating. Despite the advantageous properties, hydrogen combustion is challenging and its implementation in gas turbine applications is not straightforward. A premixed approach reduces the NOx formation due to lower combustion temperatures, but a significant risk of flashback is induced by the enhanced flame speed. Furthermore, the use of premixed combustion is an option in case of design of a new gas turbine, but in the field of lifetime extension a non-premixed approach including hydrogen blending with more conventional fuels is reasonable given that most of the original designs adopt such technology. Unfortunately, adiabatic flame temperature for diffusive combustion can reach considerably high values, thus inducing increased thermo-mechanical stresses both for the injection system and the liner, including a remarkable growth in NOx emissions. Therefore, specific activities are necessary to correctly evaluate the impact of different levels of hydrogen blend in combustors characterized by non-premixed flames, including a reliable estimation of the NOx emissions trend to possibly implement efficient countermeasures (i.e., water injection). In the present work, a 40MW heavy-duty multi-can combustor belonging to EthosEnergy is investigated by means of steady, reactive simulations with the commercial code ANSYS® FLUENT® using a pressure-based approach. The combustor geometry is simplified by removing the casing volume, being the flow split among the holes available from a previously performed calculation on the entire volume. That simplification allows for paying more attention to the chemical kinetics and NOx formation analysis thanks to the reduced computational complexity and the increased convergence rate. The simulations include the assessment of the natural gas base load configuration together with several hydrogen blends up to 50% in volume, with a subsequent fuel flow rate reduction to meet the base load turbine inlet temperature. Combustion performance is evaluated by employing an extended reaction mechanism for natural gas developed by NUI Galway, which is characterized by the presence of alkanes up to C5. The effects of hydrogen addition in the primary zone are emphasized, including an increase in temperature and flame shortening. The computation of the NOx emissions for each fuel blend relies on the calculation of the nitrogen distribution on the converged reactive solution considering the most important formation mechanisms. Finally, the acquired data sets the basis for retrofitting guidelines in the field of hydrogen non-premixed combustion for the selected gas turbine sector.