Emissions and Flame Stability Assessment of Hydrogen Addition and Air Dilution in a Microgas Turbine Combustor Using Zero-Dimensional/One-Dimensional Modeling by Chemical Reactor Network

Abstract

Hydrogen emerges as a promising fuel for clean and sustainable electricity production when utilized in microgas turbines (mGTs). However, some challenges linked to hydrogen usage must be overcome as its high reactivity can lead to increased NOx emissions and flame instabilities. Literature suggests that combustion air humidification and/or exhaust gas recirculation (EGR) may be methods to reduce this reactivity. However, these measures are limited due to the small operating range of the mGT combustor. In this context, a 20 kWth mGT combustor is investigated under various inlet conditions to assess the effect of EGR toward increased flame stability and emission control when operating under hydrogen-enriched methane firing, using a chemical reactor network (CRN) model. The CRN modeling is a fast and low computational complexity tool to model combustion performance and emissions by representing complex reactive flow fields through idealized reactor models, leading to reduced computational costs. This study examines partial load conditions, fuel compositions, and the effect of combustion air dilution through EGR. The CRN model of the combustion process is designed based on combustion zones extracted from the main flow fields with similar thermochemical states from computational fluid dynamics (CFD) analysis, while emission predictions are experimentally validated for different operating points. Predicted NOx and CO emissions by the proposed CRN model show an acceptable agreement with the experimental data at high power loads. However, its accuracy diminishes for loads below 70% due to the variability in model tuning parameters across different loads, whereas the current model is optimized for 90% load conditions. At full load with the activation of EGR, these emissions are reduced by approximately 20%. CRN results predict that NOx emissions do not increase excessively by hydrogen addition due to the constant power operation of the mGT. Indeed, the higher reactivity of hydrogen leads to a lower fuel flow rate and, thus, leaner operating conditions. On the other hand, CO emissions decrease by 15% to 40% when adding hydrogen from 5% to 50% of the volume fraction to natural gas (NG). Finally, air dilution by EGR positively affects flame temperature and NOx emissions. Tuning CRN models that are not dependent on specific load parameters to get a better agreement with experimental data, as well as EGR/humidified air dilution effects on partial loads and hydrogen-added fuels, are subjects to future work.