The interest in ammonia as a high-density hydrogen carrier for long-term electricity storage is g... more The interest in ammonia as a high-density hydrogen carrier for long-term electricity storage is growing. A clean and efficient Combined Heat and Power (CHP) system is envisioned for power production from stored ammonia, to which Homogeneous-Charge Compression-Ignition (HCCI) engines are promising. Although recent preliminary studies showed a high equivalence ratio potential for ammonia HCCI engines, its resistance to auto-ignition forces the use of high intake temperatures, which limits the (still unknown) ammonia-HCCI power density. Moreover, the feasibility of clean and highly efficient ammonia combustion has not been demonstrated. To give a first complete insight on these various aspects, an HCCI test bench has been modified to ammonia-hydrogen operation through the use of a 22:1 effective compression ratio. A cartography of the ammonia-hydrogen load range, related efficiencies and emissions is obtained following the impact of the ammonia fuel blending ratio (from 0 to 94%), equivalence ratio (from 0.1 to 0.6), intake temperature (from 50 to 240 • C) and Exhaust Gas Recirculation. Thanks to a reduced combustion intensity, ammonia allows a 50% IMEP increase compared to neat hydrogen, while maintaining equivalent combustion efficiencies. Neat hydrogen performances were not impacted from the high compression ratio. Fuel-NO X emissions have been observed, and linearly increasing with the ammonia flow rate up to 6,000 ppm, although the EGR led to a threefold reduction of those. Still EGR negatively impacted NO 2 and unburned emissions. Below combustion temperatures of 1,400 K the production of N 2 O is suspected and 1,800 K are needed to ensure complete bulk ammonia combustion. Finally, the trade-off for the ideal ammonia-hydrogen blending ratio is discussed. As perspectives, extensive work is needed on fuel-NO X primary reduction measures and after-treatment ways. Regarding primary measures, this work suggests that boosted conditions with maximized stroke-to-bore ratios should be aimed at, to allow higher EGR rates at maintained combustion temperatures.
Internal combustion engines have been improved for many decades. Yet, complex phenomena are now r... more Internal combustion engines have been improved for many decades. Yet, complex phenomena are now resorted to, for which any optimum might be unstable: noise, low-temperature heat release timing, stratification, pollutant sweet spots, and so on. In order to make reliable statements on an improvement, one must specify the uncertainty related to it. Still, uncertainty quantification is generally missing in the piston engine experimental literature. Therefore, we detailed a mathematical methodology to obtain any engine parameter uncertainty and then used it to derive the uncertainty expressions of the physical quantities of the most generic homogeneous-charge compression–ignition research engine (mass-flow-induced mixture with Cu Hy Ox Nz Sw fuel). We then applied those expressions on an existing hydrogen homogeneous-charge compression–ignition test bench. This includes the uncertainty propagation chain from sensor specifications, user calibrations, intake control, in-cylinder processes, and post-processing techniques. Directly measured physical quantities have uncertainties of around 1%, depending on the sensor quality (e.g. pressure, volume), but indirectly measured quantities relying on modelled parameters have uncertainties higher than 5% (e.g. wall heat losses, in-cylinder temperature, gross heat release, pressure rise rate). Other findings that such an analysis can bring relate, for example, to the physical quantities driving the uncertainty and to the ones that can be neglected. In the case of the homogeneous-charge compression–ignition engine considered, the effects of blow-by, bottle purity and air moisture content were found negligible; the post-processing for effective compression ratio, effective in-cylinder temperature, and top dead center offset were found essential; and the pressure and volume uncertainties were found to be the main drivers to a large extent. The obtained numeric values serve the general purpose of alerting the experimenter on uncertainty order of magnitudes. The developed methodology shall be used and adapted by the experimenter willing to study the uncertainty propagation in their setup or willing to assess the adequacy of a sensor performance.
This document summarizes the discussions held during a workshop on the energy transition, organiz... more This document summarizes the discussions held during a workshop on the energy transition, organized by several Belgian academic and industrial experts. All the questions raised above are addressed in a general way, trying to express and justify the different points of view, starting from works published in the literature.
For long-term storage, part of the excess renewable energy can be stored into various fuels, amon... more For long-term storage, part of the excess renewable energy can be stored into various fuels, among which ammonia and hydrogen show a high potential. To improve the power-to-fuel-to-power overall efficiency and reduce NOx emissions, the intrinsic properties of Low Temperature Combustion (LTC) engines could be used to convert these carbon-free fuels back into electricity and heat. Yet, ignition delay times for ammonia are not available at relevant LTC conditions. This lack of fundamental kinetic knowledge leads to uncertain ignition delay predictions by the existing ammonia kinetic mechanisms and prevents from determining optimal LTC running conditions. Using a Rapid Compression Machine (RCM), we have studied the ignition delay of ammonia with hydrogen addition (0%, 10%, and 25%vol.) under LTC conditions: low equivalence ratios (0.2, 0.35, 0.5), high pressures (43 bar and 65 bar) and low temperatures (1000 K-1100 K). This paper presents the comparison of the experimental data with simulation results obtained with five kinetic mechanisms found in the literature. It then provides a sensitivity analysis to highlight the most influencing reactions on the ignition of the ammonia-hydrogen blends. The obtained range of ignition delays for pure ammonia and for the ammonia-hydrogen blends prove their suitability for LTC engines. Still the hydrogen addition must be greater than 10%vol. to produce a significant promotion of the ignition delay. The two best performing mechanisms still predict too long ignition delays for pure ammonia, while the delays become too short for ammonia-hydrogen blends. A third mechanism captures correctly the relative influence of hydrogen addition, but is globally over-reactive. Through a sensitivity analysis, H 2 NO has been identified as the main cause for the under-reactive pure ammonia kinetics and N 2 H x has been identified as the main cause for globally over-reactive ammonia-hydrogen mechanisms.
Ammonia and hydrogen can be produced from water, air and excess renewable electricity (Power-to-f... more Ammonia and hydrogen can be produced from water, air and excess renewable electricity (Power-to-fuel) and are therefore a promising alternative in the transition from fossil fuel energy to cleaner energy sources. An Homogeneous-Charge Compression-Ignition (HCCI) engine is therefore being studied to use both fuels under a variable blending ratio for Combined Heat and Power (CHP) production. Due to the high auto-ignition resistance of ammonia, hydrogen is required to promote and stabilize the HCCI combustion. Therefore the research objective is to investigate the HCCI combustion of varying hydrogen-ammonia blending ratios in a 16:1 compression ratio engine. A specific focus is put on maximizing the ammonia proportion as well as minimizing the NOx emissions that could arise from the nitrogen contained in the ammonia. A single-cylinder, constant speed, HCCI engine has been used with an intake pressure varied from 1 to 1.5 bar and with intake temperatures ranging from 428 to 473 K. Stable combustion was achieved with up to 70 %vol. ammonia proportion by increasing the intake pressure to 1.5 bar, the intake temperature to 473 K, and the equivalence ratio to 0.28. From pure hydrogen to 60 %vol. ammonia proportion, the combustion efficiency only lost 0.6 points. Pure hydrogen Indicated Mean Effective Pressure (IMEP) was limited to 2.7 bar to avoid ringing (i.e. too high pressure rise rate) but blended with ammonia the IMEP safely reached 3.1 bar. For pure hydrogen, NOx emissions were below 6 ppm. For hydrogen-ammonia blends, NOx were between 750 and 2000 ppm. Exhaust Gas Recirculation (EGR) operations significantly reduced NOx emissions through a reduced oxygen availability but with a noticeable negative effect on combustion efficiency due to lower in-cylinder temperatures. Moreover, performed simulations showed the production of significant N2O quantities for combustion temperatures under 1400 K. Ammonia showed to be an effective fuel for HCCI conditions and EGR revealed itself as a promising NOx reducing technique through a decreased oxygen availability. Still, further effort is required when using EGR to keep the combustion temperature above 1400 K to maintain good combustion efficiencies and avoid N2O production.
In the frame of power-to-fuels, the present study focuses on the use of the simple storage fuels ... more In the frame of power-to-fuels, the present study focuses on the use of the simple storage fuels that are hydrogen, ammonia, methane and methanol for power and heat production with Homogeneous-Charge Compression-Ignition (HCCI) engines. Therefore, a HCCI 0-Dimensional model able to predict auto-ignition timings was used to determine if a single engine design could be used to burn the four fuels and their blends. This paper studies the influence of the intake conditions (equivalence ratio, temperature, and pressure) and the compression ratio on the fuel auto-ignition resistances. Results show that a single engine design is suitable for the four fuels provided that a sufficient intake temperature is reached for each fuel. Engine intake temperature control is showed to have a sufficient accuracy for the ignition timing control and therefore for the maximisation of the Indicated Mean Effective Pressure (IMEP). Fuel blending has non-linear promoting effects for hydrogen and methanol addition into ammonia and methane, respectively. A 10%vol. content of the promoter in the blend allows for a 25% increase in the IMEP through a reduction of the required intake temperature. Yet the generally low IMEP requires further work on increased equivalence ratios and the ringing risks associated with them.
The interest in ammonia as a high-density hydrogen carrier for long-term electricity storage is g... more The interest in ammonia as a high-density hydrogen carrier for long-term electricity storage is growing. A clean and efficient Combined Heat and Power (CHP) system is envisioned for power production from stored ammonia, to which Homogeneous-Charge Compression-Ignition (HCCI) engines are promising. Although recent preliminary studies showed a high equivalence ratio potential for ammonia HCCI engines, its resistance to auto-ignition forces the use of high intake temperatures, which limits the (still unknown) ammonia-HCCI power density. Moreover, the feasibility of clean and highly efficient ammonia combustion has not been demonstrated. To give a first complete insight on these various aspects, an HCCI test bench has been modified to ammonia-hydrogen operation through the use of a 22:1 effective compression ratio. A cartography of the ammonia-hydrogen load range, related efficiencies and emissions is obtained following the impact of the ammonia fuel blending ratio (from 0 to 94%), equivalence ratio (from 0.1 to 0.6), intake temperature (from 50 to 240 • C) and Exhaust Gas Recirculation. Thanks to a reduced combustion intensity, ammonia allows a 50% IMEP increase compared to neat hydrogen, while maintaining equivalent combustion efficiencies. Neat hydrogen performances were not impacted from the high compression ratio. Fuel-NO X emissions have been observed, and linearly increasing with the ammonia flow rate up to 6,000 ppm, although the EGR led to a threefold reduction of those. Still EGR negatively impacted NO 2 and unburned emissions. Below combustion temperatures of 1,400 K the production of N 2 O is suspected and 1,800 K are needed to ensure complete bulk ammonia combustion. Finally, the trade-off for the ideal ammonia-hydrogen blending ratio is discussed. As perspectives, extensive work is needed on fuel-NO X primary reduction measures and after-treatment ways. Regarding primary measures, this work suggests that boosted conditions with maximized stroke-to-bore ratios should be aimed at, to allow higher EGR rates at maintained combustion temperatures.
Internal combustion engines have been improved for many decades. Yet, complex phenomena are now r... more Internal combustion engines have been improved for many decades. Yet, complex phenomena are now resorted to, for which any optimum might be unstable: noise, low-temperature heat release timing, stratification, pollutant sweet spots, and so on. In order to make reliable statements on an improvement, one must specify the uncertainty related to it. Still, uncertainty quantification is generally missing in the piston engine experimental literature. Therefore, we detailed a mathematical methodology to obtain any engine parameter uncertainty and then used it to derive the uncertainty expressions of the physical quantities of the most generic homogeneous-charge compression–ignition research engine (mass-flow-induced mixture with Cu Hy Ox Nz Sw fuel). We then applied those expressions on an existing hydrogen homogeneous-charge compression–ignition test bench. This includes the uncertainty propagation chain from sensor specifications, user calibrations, intake control, in-cylinder processes, and post-processing techniques. Directly measured physical quantities have uncertainties of around 1%, depending on the sensor quality (e.g. pressure, volume), but indirectly measured quantities relying on modelled parameters have uncertainties higher than 5% (e.g. wall heat losses, in-cylinder temperature, gross heat release, pressure rise rate). Other findings that such an analysis can bring relate, for example, to the physical quantities driving the uncertainty and to the ones that can be neglected. In the case of the homogeneous-charge compression–ignition engine considered, the effects of blow-by, bottle purity and air moisture content were found negligible; the post-processing for effective compression ratio, effective in-cylinder temperature, and top dead center offset were found essential; and the pressure and volume uncertainties were found to be the main drivers to a large extent. The obtained numeric values serve the general purpose of alerting the experimenter on uncertainty order of magnitudes. The developed methodology shall be used and adapted by the experimenter willing to study the uncertainty propagation in their setup or willing to assess the adequacy of a sensor performance.
This document summarizes the discussions held during a workshop on the energy transition, organiz... more This document summarizes the discussions held during a workshop on the energy transition, organized by several Belgian academic and industrial experts. All the questions raised above are addressed in a general way, trying to express and justify the different points of view, starting from works published in the literature.
For long-term storage, part of the excess renewable energy can be stored into various fuels, amon... more For long-term storage, part of the excess renewable energy can be stored into various fuels, among which ammonia and hydrogen show a high potential. To improve the power-to-fuel-to-power overall efficiency and reduce NOx emissions, the intrinsic properties of Low Temperature Combustion (LTC) engines could be used to convert these carbon-free fuels back into electricity and heat. Yet, ignition delay times for ammonia are not available at relevant LTC conditions. This lack of fundamental kinetic knowledge leads to uncertain ignition delay predictions by the existing ammonia kinetic mechanisms and prevents from determining optimal LTC running conditions. Using a Rapid Compression Machine (RCM), we have studied the ignition delay of ammonia with hydrogen addition (0%, 10%, and 25%vol.) under LTC conditions: low equivalence ratios (0.2, 0.35, 0.5), high pressures (43 bar and 65 bar) and low temperatures (1000 K-1100 K). This paper presents the comparison of the experimental data with simulation results obtained with five kinetic mechanisms found in the literature. It then provides a sensitivity analysis to highlight the most influencing reactions on the ignition of the ammonia-hydrogen blends. The obtained range of ignition delays for pure ammonia and for the ammonia-hydrogen blends prove their suitability for LTC engines. Still the hydrogen addition must be greater than 10%vol. to produce a significant promotion of the ignition delay. The two best performing mechanisms still predict too long ignition delays for pure ammonia, while the delays become too short for ammonia-hydrogen blends. A third mechanism captures correctly the relative influence of hydrogen addition, but is globally over-reactive. Through a sensitivity analysis, H 2 NO has been identified as the main cause for the under-reactive pure ammonia kinetics and N 2 H x has been identified as the main cause for globally over-reactive ammonia-hydrogen mechanisms.
Ammonia and hydrogen can be produced from water, air and excess renewable electricity (Power-to-f... more Ammonia and hydrogen can be produced from water, air and excess renewable electricity (Power-to-fuel) and are therefore a promising alternative in the transition from fossil fuel energy to cleaner energy sources. An Homogeneous-Charge Compression-Ignition (HCCI) engine is therefore being studied to use both fuels under a variable blending ratio for Combined Heat and Power (CHP) production. Due to the high auto-ignition resistance of ammonia, hydrogen is required to promote and stabilize the HCCI combustion. Therefore the research objective is to investigate the HCCI combustion of varying hydrogen-ammonia blending ratios in a 16:1 compression ratio engine. A specific focus is put on maximizing the ammonia proportion as well as minimizing the NOx emissions that could arise from the nitrogen contained in the ammonia. A single-cylinder, constant speed, HCCI engine has been used with an intake pressure varied from 1 to 1.5 bar and with intake temperatures ranging from 428 to 473 K. Stable combustion was achieved with up to 70 %vol. ammonia proportion by increasing the intake pressure to 1.5 bar, the intake temperature to 473 K, and the equivalence ratio to 0.28. From pure hydrogen to 60 %vol. ammonia proportion, the combustion efficiency only lost 0.6 points. Pure hydrogen Indicated Mean Effective Pressure (IMEP) was limited to 2.7 bar to avoid ringing (i.e. too high pressure rise rate) but blended with ammonia the IMEP safely reached 3.1 bar. For pure hydrogen, NOx emissions were below 6 ppm. For hydrogen-ammonia blends, NOx were between 750 and 2000 ppm. Exhaust Gas Recirculation (EGR) operations significantly reduced NOx emissions through a reduced oxygen availability but with a noticeable negative effect on combustion efficiency due to lower in-cylinder temperatures. Moreover, performed simulations showed the production of significant N2O quantities for combustion temperatures under 1400 K. Ammonia showed to be an effective fuel for HCCI conditions and EGR revealed itself as a promising NOx reducing technique through a decreased oxygen availability. Still, further effort is required when using EGR to keep the combustion temperature above 1400 K to maintain good combustion efficiencies and avoid N2O production.
In the frame of power-to-fuels, the present study focuses on the use of the simple storage fuels ... more In the frame of power-to-fuels, the present study focuses on the use of the simple storage fuels that are hydrogen, ammonia, methane and methanol for power and heat production with Homogeneous-Charge Compression-Ignition (HCCI) engines. Therefore, a HCCI 0-Dimensional model able to predict auto-ignition timings was used to determine if a single engine design could be used to burn the four fuels and their blends. This paper studies the influence of the intake conditions (equivalence ratio, temperature, and pressure) and the compression ratio on the fuel auto-ignition resistances. Results show that a single engine design is suitable for the four fuels provided that a sufficient intake temperature is reached for each fuel. Engine intake temperature control is showed to have a sufficient accuracy for the ignition timing control and therefore for the maximisation of the Indicated Mean Effective Pressure (IMEP). Fuel blending has non-linear promoting effects for hydrogen and methanol addition into ammonia and methane, respectively. A 10%vol. content of the promoter in the blend allows for a 25% increase in the IMEP through a reduction of the required intake temperature. Yet the generally low IMEP requires further work on increased equivalence ratios and the ringing risks associated with them.
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