Periodic Reporting for period 2 - GREENBLUE (Greenhouse Gas and Pollutant Emission Reductions using Plasma-Assisted Combustion for a Blue Planet)
Reporting period: 2023-05-01 to 2024-10-31
To fight climate change, we must urgently reduce the CO2 emissions caused by fossil-fuel combustion, which represents today over 80% of the primary energy production. Clean electrified solutions are on the horizon but are unlikely to reach commercial development before 2040. Novel CO2-neutral (biofuels) or CO2-free (H2) combustion technologies are widely considered, but these technologies face increasingly stringent regulations on pollutant emissions, in particular nitric oxides and carbon monoxide. Nitric oxides emitted at the ground level cause major health issues and are responsible to more than 25,000 deaths each year. At higher altitudes, they negatively affect the ozone layer and cause acid rain. To reduce pollutants, the strategy is to use low-temperature flames. However, these flames are prone to instabilities and extinction, thus causing safety issues. Plasma-assisted combustion (PAC) is a promising method to stabilize low-temperature flames thanks to the extraordinary ability of plasma discharges to efficiently produce combustion-enhancing radicals. Today, however, their effects on pollutants are poorly understood and their scalability to industrial combustors remains to be proven.
Our goal is to bring PAC to the level of maturity needed to make it practical on real combustion devices. For this, we will first elucidate the thermochemical mechanisms of plasma stabilization in CH4- and H2-air flames and their impact on pollutant emissions. This will require measuring the rates of poorly known reactions involving excited electronic states of molecules with advanced femtosecond optical diagnostics. With this knowledge, we will explore two novel strategies to minimize pollutants. We will then develop a robust and versatile multi-physics model to predict PAC effects in large-scale combustors. The final challenge will be to demonstrate for the first time a stable, low NOx, hydrogen/air flame in a combustor representative of aircraft engines. Beyond combustion, this project will open novel ways to better predict, control, and enhance chemical processes in applications such as hydrogen production, CO2 conversion, bio-decontamination, or materials synthesis
Our goal is to bring PAC to the level of maturity needed to make it practical on real combustion devices. For this, we will first elucidate the thermochemical mechanisms of plasma stabilization in CH4- and H2-air flames and their impact on pollutant emissions. This will require measuring the rates of poorly known reactions involving excited electronic states of molecules with advanced femtosecond optical diagnostics. With this knowledge, we will explore two novel strategies to minimize pollutants. We will then develop a robust and versatile multi-physics model to predict PAC effects in large-scale combustors. The final challenge will be to demonstrate for the first time a stable, low NOx, hydrogen/air flame in a combustor representative of aircraft engines. Beyond combustion, this project will open novel ways to better predict, control, and enhance chemical processes in applications such as hydrogen production, CO2 conversion, bio-decontamination, or materials synthesis
We have performed the first quantitative comparisons between three-dimensional Large-Eddy Simulations (LES) of a plasma-assisted, turbulent, methane-air flame in the Mini-PAC burner (15 kW, 1 atm) and temporally resolved measurements of flame evolution, radical species (OH), and temperature. The plasma was produced by Nanosecond Repetitively Pulsed (NRP) discharges. The simulations were made with the phenomenological PAC model developed in our previous work (Castela et al., Comb. & Flame, 166, 133-147, 2016) implemented in the YALES2 LES package. The remarkable agreement between experiments and LES confirms that our approach for modeling PAC with the phenomenological model of Castela et al. is a well-grounded methodology to quantify the effects of plasmas in flames, even in burnt gas mixtures.
We have also extended the model of Castela et al. to any mixture of N2 with species such as CO2, H2O, H2, CH4, and NO (Ph.D. thesis of V. Blanchard, Univ. Paris Saclay, 2023). This new model is analytical and thus offers the same convenience of implementation into LES codes as the phenomenological model. Comparisons with our experimental datasets acquired with CH4-air and H2-air flames are in progress.
We have succeeded in decreasing by 60% the lean blow-off limit of a high power (150 kW) staged multipoint combustor working with CH4-air at 1 atm, for very low plasma power input (0.2% of flame thermal power), and with NOx emissions comparable to those of the leanest flame without plasma. This low plasma power is achieved thanks to an optimized nanosecond pulsing pattern (discharge tailoring). Although the combustion efficiency is only 40% near blow-off, the stabilized flame is still useful to prevent engine extinction during fast transient flight regimes (Blanchard et al. App. Energy Comb. Sci. 15, 100158, 2023) and (Blanchard et al. J. Eng. Gas Turbines Power. 146(8): 081012, 2024).
For hydrogen-air flames, we have decreased the lean blow-off limit by 10% in a laboratory-scale burner, SICCA (4 kW, 1 atm). The measured NOx emissions are comparable with those of the leanest H2-air flame without plasma and remain below regulatory limits (Perrin-Terrin et al. Proc. Comb. Inst. 40, 105546, 2024). This is the first time that NRP discharges are shown to have a marked effect on hydrogen-air flames. The experimental dataset will be used to test numerical simulations of PAC in H2-air flames.
We have also extended the model of Castela et al. to any mixture of N2 with species such as CO2, H2O, H2, CH4, and NO (Ph.D. thesis of V. Blanchard, Univ. Paris Saclay, 2023). This new model is analytical and thus offers the same convenience of implementation into LES codes as the phenomenological model. Comparisons with our experimental datasets acquired with CH4-air and H2-air flames are in progress.
We have succeeded in decreasing by 60% the lean blow-off limit of a high power (150 kW) staged multipoint combustor working with CH4-air at 1 atm, for very low plasma power input (0.2% of flame thermal power), and with NOx emissions comparable to those of the leanest flame without plasma. This low plasma power is achieved thanks to an optimized nanosecond pulsing pattern (discharge tailoring). Although the combustion efficiency is only 40% near blow-off, the stabilized flame is still useful to prevent engine extinction during fast transient flight regimes (Blanchard et al. App. Energy Comb. Sci. 15, 100158, 2023) and (Blanchard et al. J. Eng. Gas Turbines Power. 146(8): 081012, 2024).
For hydrogen-air flames, we have decreased the lean blow-off limit by 10% in a laboratory-scale burner, SICCA (4 kW, 1 atm). The measured NOx emissions are comparable with those of the leanest H2-air flame without plasma and remain below regulatory limits (Perrin-Terrin et al. Proc. Comb. Inst. 40, 105546, 2024). This is the first time that NRP discharges are shown to have a marked effect on hydrogen-air flames. The experimental dataset will be used to test numerical simulations of PAC in H2-air flames.
As detailed below, the work performed to date has advanced the state-of-the-art in plasma-assisted stabilization of methane-air and hydrogen-air flames. A broad outline for future work is also presented.
- First, we demonstrated a significant extension (60%) of the LBO limit of an atmospheric methane-air flame in a model staged combustor representative of lean-premixed aeronautical engines with a minimal NOx penalty. We are currently exploring how to further increase the combustion efficiency, which was limited to 40% near blow-off, by modifying the discharge parameters and electrode locations. We will also extend this work to blends of CH4 and H2, and progressively to hydrogen-air flames.
- Second, we have performed the first demonstration of LBO extension with H2-air flames in a swirled laboratory-scale burner, also with minimal NOx penalty.
In both experiments, we have explored NRP pulse tailoring strategies, which resulted in lower NOx penalties and a reduced plasma power, by a factor 5 to 10 compared to continuous pulsing. This marks considerable progress over previous studies that had a strong NOx penalty induced by plasma discharges.
- Third, we have performed massive LES simulations of a 3D plasma-stabilized turbulent methane-air flame and we have validated the numerical results against quantitative measurements. This work will be continued with LES simulations using a new analytical PAC model developed in this work, which used to explore optimal PAC strategies, with the objective of minimizing the required plasma power and the NOx emissions. Both experiments and simulations will be conducted for a variety of fuels, including methane-air as a surrogate for Sustainable Aviation Fuels, as well as blends of methane and air, and finally hydrogen.
- Fourth, experimental work is in progress to measure the rate coefficients of important reactions in PAC, in particular the plasma-driven reactions related to NOx chemistry. These measurements are being performed with optical diagnostics including Quantitative Optical Emission Spectroscopy, Laser Induced Fluorescence, Spontaneous Raman Scattering, Hybrid Femto-Picosecond CARS, and femtosecond TALIF.
- First, we demonstrated a significant extension (60%) of the LBO limit of an atmospheric methane-air flame in a model staged combustor representative of lean-premixed aeronautical engines with a minimal NOx penalty. We are currently exploring how to further increase the combustion efficiency, which was limited to 40% near blow-off, by modifying the discharge parameters and electrode locations. We will also extend this work to blends of CH4 and H2, and progressively to hydrogen-air flames.
- Second, we have performed the first demonstration of LBO extension with H2-air flames in a swirled laboratory-scale burner, also with minimal NOx penalty.
In both experiments, we have explored NRP pulse tailoring strategies, which resulted in lower NOx penalties and a reduced plasma power, by a factor 5 to 10 compared to continuous pulsing. This marks considerable progress over previous studies that had a strong NOx penalty induced by plasma discharges.
- Third, we have performed massive LES simulations of a 3D plasma-stabilized turbulent methane-air flame and we have validated the numerical results against quantitative measurements. This work will be continued with LES simulations using a new analytical PAC model developed in this work, which used to explore optimal PAC strategies, with the objective of minimizing the required plasma power and the NOx emissions. Both experiments and simulations will be conducted for a variety of fuels, including methane-air as a surrogate for Sustainable Aviation Fuels, as well as blends of methane and air, and finally hydrogen.
- Fourth, experimental work is in progress to measure the rate coefficients of important reactions in PAC, in particular the plasma-driven reactions related to NOx chemistry. These measurements are being performed with optical diagnostics including Quantitative Optical Emission Spectroscopy, Laser Induced Fluorescence, Spontaneous Raman Scattering, Hybrid Femto-Picosecond CARS, and femtosecond TALIF.