Ahmed F. Ghoniem

Redox thermochemical processes are explored for their potential in cost-effective hydrogen production and high-temperature industrial heat supply, addressing global decarbonization challenges. For water splitting, two-step redox cycles operating above 1000 °C, such as those involving ceria (CeO2), face material stability and handling complexities. Alternatively, multi-step cycles using non-toxic, earth-abundant materials like manganese oxide and sodium carbonate (Mn3O4/Na2CO3) enable operation at lower temperatures, with continuous cycling demonstrated at approximately 850 °C for thermal reduction. Innovations in these systems include material development (e.g., ceria-ferrite composites) and advanced system designs that integrate reactor trains, regenerative heat transfer, and efficient oxygen removal (e.g., electrochemical/thermochemical pumps), with modeling indicating potential solar-to-fuel efficiency improvements from 10% to nearly 40%. In thermochemical energy storage, magnesium-manganese oxide systems store renewable electricity as high-temperature heat, delivering up to 1450 °C for industrial applications. This approach achieves fast charging (within 4 hours), high energy density (~2400 MJ/m³), and a 95% round-trip heat efficiency, demonstrating stability over 1000 cycles. Overall, thermal energy storage is critical for enabling continuous, 24/7 operation and reducing costs by leveraging less expensive heat sources. These processes are significant for producing hydrogen and for the synthesis of liquid fuels like syngas and methanol from water and captured carbon dioxide, contributing to broader decarbonization efforts. Welcome from the Chair Thermochemical Water Splitting: Then, Now and Future The Development of Thermochemical Energy Storage Technology for Scalable High Temperature Industrial Heat Applications Thermochemical Syngas Production using Redox Cycles Discussion

Biomass torrefaction is a thermochemical pretreatment that can be used to improve biomass properties, contributing to the energy densification of biomass or increasing its grindability, burning and gasification characteristics, porous structure, etc. While it is typically carried out under inert conditions, oxygen-lean torrefaction of biomass can reduce the process cost and complexity. This work proposes a novel extended 2-step kinetics mechanism capable of accurately predicting the evolution of the chemical composition of torrefied biomass. The model parameters are obtained from data collected from an extensive experimental campaign in which the chemical composition of torrefied biomass was measured at different temperatures, residence times, and oxygen fractions. The carbon mass fraction of the torrefied products was found to increase by up to 50 % under severe inert torrefaction conditions, i.e., high temperatures and long residence times, whereas the hydrogen fraction decreased. In the presence of oxygen during torrefaction, the carbon and hydrogen contents in the solid residue decrease compared with those produced under inert torrefaction. The extended 2-step model can also be used to determine the high heating value and energy densification of the torrefied biomass, with the latter ranging from 1 to 1.4 for the operating conditions tested. • Torrefied biomass chemical composition was modeled by a 2-step kinetics mechanism. • The 2-step model is valid for both inert and oxidative torrefaction of biomass. • Torrefaction tests were run varying temperature, residence time and oxygen fraction. • Model deviations are below 1 % for carbon and 0.2 % for hydrogen concentration. • Estimated energy densification reaches values of 1.4 at 300 °C during 120 min.

Solar thermochemical hydrogen (STCH) produced by heat-driven water-splitting is a promising route for producing green hydrogen and other zero-emission synfuels. However, the efficiency of STCH must be dramatically increased for it to make an impact on decarbonization efforts. We have previously presented a novel Reactor Train System (RTS) for significantly increasing the efficiency of STCH by employing heat recovery from the redox material and efficient gas exchange processes. In this paper we present a higher-fidelity model for the RTS that accommodates the slow heat diffusion through the STCH redox material. For this purpose, a novel method is introduced for transient modelling of radiative heat in participating media. This method, called GREENER: Generalized Radiation Exchange Factors and Net Radiation, combines the accuracy of Monte Carlo Ray Tracing with the low computational cost of the P1 or Rosseland diffusion approximations. Along with STCH, GREENER has application for modelling volumetric solar receivers, high temperature heat recovery systems like heat exchangers and regenerators, and packed bed reactors. Using the GREENER method, the RTS counterflow radiative heat exchanger is shown to achieve heat recovery effectiveness greater than 70%. The performance of non-uniform porous redox morphologies is evaluated, and high-performing configurations are identified.

In this work, we performed a preliminary investigation on the redox behaviour of Sr2FeMo0.6Ni0.4O6-δ (SFMN) double perovskite in H2-H2O and CH4-H2O redox cycles in order to explore the potential use of this oxide as an Oxygen Carrier (OC) in fuel-assisted Chemical Looping Water Splitting (CLWS) processes driven by concentrated solar energy. The results were compared with our previous findings on the Reverse Water Gas Shift Chemical Looping (RWGS-CL) reaction. The improvement in performance due to the bimetallic exsolution on the OC surface is observed. This OC exhibits interesting activity and stability over CH4-assisted CLWS cycling. Future investigations are planned to examine the structural transformations that might impact the redox behaviour of this material in water splitting processes.

This researchg investigates the decarbonization of India’s electricity grid using ammonia in power plants. It focuses on ammonia produced in Western Australia and transported to India, co-fired with high rank coal, and compared with power plants utilizing carbon capture and sequestration (CCS). The study assesses the overall costs and the life cycle greenhouse gas (LC GHG) emissions for both new plants and retrofits. For 20% gray, blue, and green ammonia, the levelized cost of electricity is 86, 89, 125 $/MWh, with corresponding LC GHG emissions of 1,234, 1,079, and 1,062 kg CO2e/MWh. Co-firing with green ammonia, though more expensive than blue ammonia, yields lower CO2 emissions. Conversely, reducing the same amount of direct CO2 emission via CCS costs $84/MWh and a LC GHG emission of 1,227 kg CO2e/MWh. While CCS is cheaper, it results in higher LC GHG. There is a trade-off between cost and emissions across the strategies. Under scenarios with low capacity factors or reduced ammonia production costs, coal-ammonia co-firing could become more economical and greener than the CCS. This study provides quantitative insights for policymakers and project developers. However, it is crucial for decision-makers to consider several factors: (1) the potential impact of social resistance to CCS; (2) the time required for large-scale commercialization of CCS technology, which is expected to be significantly longer than the implementation time for a coal-ammonia co-firing decarbonization strategy; (3) the potential of either CCS or ammonia-coal co-firing strategy to enhance India’s electricity mix, thus contributing to energy security.

Abstract We have previously proposed the Reactor Train System (RTS) for efficient solar thermochemical hydrogen (STCH) production, which utilizes a counter-flow radiative heat exchange system with moving reactors. RTS reactors are heated with indirect infrared radiation, which is markedly different from current experimentally demonstrated solar receiver-reactors. In this work we present a design study of a prototype reactor, including a high-fidelity heat transfer model with component dimensions informed by mechanical stress modeling. This model was used to subject different variants of the reactor to a heating cycle, and reactor performance metrics such as hydrogen productivity and component temperatures were compared. We report that thermal performance improves when smaller structural components are used, and when active cooling losses are reduced. Our findings show that indirectly heated reactors have unique design considerations, and conventional concentrated solar receivers perform poorly even with a high-temperature window. Strategies for improving the performance of indirectly irradiated thermochemical reactors are identified.

Abstract When using nanosecond repetitively pulsed discharges to actuate on dynamic combustion instabilities, the environment the discharge is created in is unsteady and changing on the timescale of the combustion processes. As a result, individual discharge pulses are triggered in a background gas that evolves at the timescale of combustion dynamics, and pulse-to-pulse variations may be observed during the instability cycle. Prior work has studied nanosecond pulsed discharges in pin-to-ring configurations used to control instabilities in lean-operating swirl-stabilized combustors, and observed variable discharge behavior. The focus of this work is on characterizing how the pulse-to-pulse discharge morphology, energy deposition, and actuation authority, evolve during the combustion instability cycle. This has important implications for designing effective plasma-assisted combustion control schemes. The discharge is observed in two distinct modes, a streamer corona and a nanosecond spark, with the occurrence of each regime directly linked to the phase of the combustor instability. Variation of pulse repetition frequency affects the total fraction of pulses in each mode, while variation of voltage affects the onset of the nanosecond spark mode. The transitions are described in terms of ratios of the relevant combustion and plasma timescales and the implications of this coupled interaction on the design of an effective control scheme is discussed.

This study investigates the structural evolution and redox characteristics of the double perovskite Sr2FeMo0.6Ni0.4O6-δ (SFMN) during hydrogen (H2) and carbon dioxide (CO2) redox cycles and explores the material performance in the Reverse Water-Gas Shift Chemical Looping (RWGS-CL) reaction. In-situ and ex-situ X-Ray Diffraction (XRD) and High-Resolution Transmission Electron Microscopy (HRTEM) studies reveal that H2 reduction at temperatures above 800 °C leads to the exsolution of bimetallic Ni-Fe alloy particles and the formation of a Ruddlesden-Popper (RP) phase. A core–shell structure with Ni-Fe core and a perovskite oxide shell is formed with subsequent redox cycles, and the resulting material exhibits better performance and high stability in the RWGS-CL process. Thermogravimetric (TGA) and Temperature Programmed Reduction (TPR) and Oxidation (TPO) analyses show that the optimal reduction and oxidation temperatures for maximizing the CO yield are around 850 °C and 750 °C respectively, and that the cycled material is able to work steadily under isothermal conditions at 850 °C.

The aim of this work is to investigate the effect of exhaust gas recirculation (EGR: water vapor and CO2), with and without O2 enrichment, on non-premixed turbulent flames stabilized on a swirl burner. The motivation includes CO2 capture applications using O2 and CO2, combustion of biogas that containing CO2 and the use of EGR or H2O in certain industrial applications to reduce pollutant emissions. Experiments were carried out on a coaxial swirl burner placed in a combustion chamber of 25 kW nominal power. The oxidant (air-O2, + H2O, + CO2) is introduced in the annular part though a swirler. The fuel (CH4) is fed though the central tube and injected radially at the exit section. The study focuses on laminar burning velocity, pollutant emissions, flame stability, and flow fields measurements with different fractions of O2, H2O and CO2 in the mixture. The fraction of diluents is varied from 0 to 20%, O2 concentration from 21 to 25% (in vol.) and the swirl number from 0.8 to 1.4. Different measurements are recorded: OH* chemiluminescence to locate the flame front, Stereo-PIV to analyze the flow field, pollutant emissions analysis (NOx and CO) and temperatures in the combustion chamber. Results show that dilution significantly influences flame characteristics. Dilution increases the lift-off height and reduces flame stability especially with high fractions (16–20%), whereas O2 enrichment decreases lift-off height and enhances flame stability. The increase dilution reduces NOx and increases CO emissions. Stereo-PIV measurements reveals the turbulent coherent structure of the swirling flow as well as the effect of dilution on the corresponding axial and tangential velocities. The effect of dilution on the underlying laminar burning velocity were determined by 1D calculation using COSILAB with GRI3.0 mechanism.

View Video Presentation: https://doi.org/10.2514/6.2023-3805.vid There is a growing interest in using ammonia as a fuel in power and long-distance transportation such as shipping and aviation. Thermochemical and combustion characteristics of ammonia are significantly different from those of conventional fuels such as natural gas, diesel and jet fuels. In this work we investigate some fuel blends’ combustion stability characteristics to better understand their compatibility with existing combustion systems. Tests are done in a well characterized 14-kW swirl-stabilized burner using CH4/NH3 mixtures with the goal of examining the impact of the blend ratio on thermoacoustic instabilities and flame macrostructures. Increasing the percentage of ammonia makes the combustor more stable thermoacoustically, shifting the operating regimes to those associated with longer and weaker flames with a lower blowoff margin. The measured SPL curves as function of the equivalence ratio for different blend ratio collapse onto as single line when the data is plotted as a function of flame time scale determined by the extinction strain rate. This is consistent with previous results for, e.g., methane-hydrogen blends, operating under stable and unstable conditions. For weaker flames largely operating under stable conditions, a flame time scale determined by the laminar burning velocity is also possible. These results inform how existing combustors can be retrofitted to operate with hydrogen carriers such as ammonia.

Abstract The aim of this work is to investigate the effect of exhaust gas recirculation (EGR: water vapor and CO 2 ), with and without O 2 enrichment, on non-premixed turbulent flames stabilized on a swirl burner. The motivations include CO 2 capture applications using O 2 and CO 2, combustion of biogas that contains CO 2 and the use of EGR or H 2 O in certain industrial applications to reduce pollutant emissions. Experiments were carried out on a coaxial swirl burner placed in a combustion chamber of 25 kW of nominal power. The oxidant (air-O 2 , +H 2 O, +CO 2 ) is introduced in the annular part though a swirler. The fuel (CH 4 ) is fed though the central tube and injected radially at the exit section. The study focused on laminar burning velocity, pollutant emissions, flame stability, and flow fields measurements with different fractions of O 2 , H 2 O and CO 2 in the mixture. The fraction of diluents varied from 0 to 20%, O 2 concentration from 21 to 25% (in vol.) and the swirl number from 0.8 to 1.4. Different measurements were recorded: OH* chemiluminescence to locate the flame front, Stereo-PIV to analyze the flow field, pollutant emissions analysis (NOx and CO) and temperatures in the combustion chamber. Results show that dilution significantly influences flame characteristics. Dilution increases the lift-off height and reduces flame stability especially with high fractions (16-20%). O 2 enrichment decreases lift-off height and enhances flame stability. Increase dilution reduces NOx and increases CO emissions. Stereo-PIV measurements highlight the turbulent coherent structure of the swirling flow and the effect of dilution on axial and tangential velocities. The effect of dilution on the underlying laminar burning velocity were determined by 1D calculation using COSILAB with GRI3.0 mechanism.

This paper details the use of nanosecond repetitively pulsed discharges to attenuate combustion instabilities in a 14 kW swirl-stabilized methane/air combustor. The combustor exhibits large-amplitude pressure oscillations ranging from 1 to 4% of the mean pressure during which the flame exhibits bulk motion in each instability cycle, upstream and downstream, as revealed by high-speed chemiluminescence. Control is accomplished with an electrode comprising a pin anode at the centerline of the combustor, allowing a nanosecond spark to be generated in a region spanning close to the flame base, through the shear layers of the swirling flow and ending at the metallic combustor wall. The discharges are generated using 20 kV, 9 kHz pulses; and they correspond to about 120 W of mean power. This results in a suppression of the peak amplitude of the pressure oscillations by a factor of two to four, and 5 dB in the rms value. Using phase-averaged visualizations of the flame with and without plasma, we detail the sequence of flame motion in the course of the instability. With the plasma active, this reveals significant interactions between the flame and the plasma during the suppression. Finally, we present a state-space model of the thermoacoustic system, and we demonstrate open-loop control of the instabilities.

Combustor. The combustor includes an axially symmetric tube along with means for introducing fuel and air into the tube. A swirler is disposed within the tube to impart rotation in a first direction to the air/fuel mixture. A plurality of holes downstream of the swirler are disposed around the tube and offset at an angle relative to an inward normal to the tube wall. Air is injected through the offset holes to impart rotation to the air/fuel mixture in a second direction opposite to the first direction. A combustion chamber having a diameter larger than that of the tube receives and burns the air/fuel mixture from the tube.

View Video Presentation: https://doi.org/10.2514/6.2023-1061.vid The precessing vortex core (PVC) is a self-excited flow oscillation in swirl nozzle combustors that can influence combustor operation by impacting unsteady flame dynamics, fuel-air mixing and emissions. The flow configuration studied is a swirl nozzle mounted in a dump combustor with an axial swirler. The bulk flow velocity at the nozzle exit plane is kept fixed at Ub = 8 m/s (Re ~ 20,000) and the swirl number, S=0.67. Our prior experimental study has shown that PVC oscillations can be rendered intermittent when centrebodies are introduced. Time averaged flow states needed for linear stability and resolvent analysis are determined from LES for two cases, with and without a centrebody respectively. For the case without a centrebody, linear stability analysis reveals a marginally stable PVC mode with a flow feedback region("wavemaker") located at the upstream end of the vortex breakdown bubble(VBB). This shows that the wake behind the centrebody interferes with flow feedback causing PVC suppression. Resolvent analysis on the case with a centrebody shows peak gain and low rank behaviour close to the intermittent PVC frequency determined from experiments. The spatial flow oscillation amplitude structure of the output mode confirms that the response to the input is a PVC. The input mode structure shows stochastic turbulent fluctuations whose helical component imposes a radial deflection of the VBB away from the centreline at its upstream end and causes the nominally merged centrebody wake and the VBB to separate. This results in the emergence of intermittent PVC oscillations.