Recent Submissions

  • Effect of fuel reactivity on flame properties of a low-swirl burner

    Saqib Akhtar, Muhammad; Shahsavari, Mohammad; Ghosh, Anupam; Wang, Bing; Hussain, Zahid; Rao, Zhuming (Experimental Thermal and Fluid Science, Elsevier BV, 2022-11-21) [Article]
    The present paper examines the capability of low swirl flows in stabilizing low calorific value fuels. To such an aim, CO2 gas is used to adjust the reactivity of CH4/air mixtures. The results show that increases in CO2 percentage in the fresh mixture result in shrinking the burner stability map by moving the lean and rich blowout limits toward the stoichiometric condition. The effects of CO2 addition on rich blowout limits are more intense than the lean blowout limits. For instance, the equivalence ratio associated with the lean blowout increases by 0.23, when the discharge flow velocity from the burner rises from 1.5 to 5 m/s in the 40 % CO2 diluted case. Under a similar augmentation in the burner discharge velocity, the rich blowout limit reduces by more than 0.5 in the 40 % CO2 diluted case. Unlike the lean blowout, the rich blowout limit responds non-monotonically to the discharge flow velocity from the burner for both undiluted and diluted mixtures. For the 50 % CO2 diluted case, the low swirl burner operates efficiently at low discharge velocities (1.5 to 3.5 m/s), but it blows out at high discharge velocities (3.5 to 9 m/s). Furthermore, unsteady features of low swirl flame near blowout limits are studied in detail. Moreover, strong curvy structures appear at the flame boundary prior to the flame flashback. However, such structures diminish occasionally and specifically as the flame approaches the flashback. Some deterministic features are more obvious before the flame blowout than those before the flame flashback.
  • Turbulent flame speed of NH3/CH4/H2/H2O/air-mixtures: Effects of elevated pressure and Lewis number

    Wang, Shixing; Elbaz, Ayman M.; Wang, Guoqing; Wang, Zhihua; Roberts, William L. (Combustion and Flame, Elsevier BV, 2022-11-13) [Article]
    This study investigated the turbulent flame speed (ST) of NH3/CH4/H2/air mixtures subjected to differential-diffusion effect characterized by sub-unity Lewis number (Le) at elevated pressures (1 and 5 atm), temperature (373 K), lean equivalence ratios (ϕ = 0.6–0.91) with and without 10% volumetric H2O dilution. The experiments were conducted in a fan-stirred constant volume combustion vessel, with a homogeneous turbulence intensity of u′ = 0 - 2.34 m/s. Schlieren imaging was used to derive the instantaneous turbulent flame speed. Morphology analysis shows that at atmospheric-pressure conditions, the laminar diffusional-thermal unstable flame surface is subjected to cellular wrinkling, this instability is increased at elevated-pressure conditions. Whereas, external turbulence will aggravate the wrinkling but the increase in turbulence intensity suppressed differential-diffusion effects on the flame structure. Flame speed development at sub-unity Le cases show differential diffusion effect and propagates faster than unity Le cases even though they have similar laminar flame speed. Then, the turbulent spherical flame speeds are scaled with a power-law fitting of flame Reynolds number, ReT,flame. The normalized turbulent propagation speed (ST/SL) is much higher for the sub-unity Le flames but it decreases as hydrogen mole fraction (XH2) increases in the fuel mixture. This was explained by the synergistic effect of differential diffusion and turbulence. Water dilution and elevated pressure via increase of Karlovitz number (Ka) and turbulent ReT,flow affect the ST/SL increasing. At sufficiently low Damköhler numbers, Da, a ratio of normalized turbulent flame speed ST/SL is mainly controlled by ReT and scales as a half power law, in line with the classical hypothesis by Damköhler. Additional turbulent stretch modification with Ka or Da can correlate with different hydrogen content conditions, this is because the independent pair of scaling parameters from a set of (u′/SL, lT/lf, Da, Ka, ReT) is considered. The scaling parameter ((u′/SL)α(lT/lf)β(Le)γ) is insensitive to Lewis number, because of the dominance of relative turbulent intensity and turbulent length scale.
  • Computational thermochemistry of oxygenated polycyclic aromatic hydrocarbons and relevant radicals

    Wang, Tairan; Yalamanchi, Kiran K.; Bai, Xin; Liu, Shuyuan; Li, Yang; Qu, Bei; Kukkadapu, Goutham; Sarathy, Mani (Combustion and Flame, Elsevier BV, 2022-11-13) [Article]
    Oxygenated polycyclic aromatic hydrocarbons (OPAHs) have attracted growing attention due to their toxicological harmfulness and significant role in soot formation. This study comprehensively investigates the thermochemistry of OPAH species and relevant radicals via quantum-chemical calculations. Temperature-dependent enthalpy of formation, entropy, and heat capacity for C5single bondC18 OPAHs (59 molecules and 33 radicals) are consistently determined at the M06–2X/6-311++G(d,p) level of theory. Considerable differences are found to be introduced by different methods when calculating electronic energies, and the G3 method outperforms the composite compound methods G3/G4/CBS-APNO. The calculated thermochemical properties from the G3 method show excellent agreement with literature data. An accurate thermochemistry database for OPAHs is thus developed. In addition, the existing group additivity (GA) method does not apply to OPAHs since the group additivity values (GAVs) derived from small hydrocarbons fail to predict large polycyclic species. Based on our dataset, GAVs are obtained from combinatorial considerations. The updated GAVs can be applied with enhanced confidence to estimate the thermochemical parameters at different temperatures for larger OPAHs where such high-accuracy quantum chemistry calculations are intractable. These thermodynamic properties and GAVs are crucial for the development of accurate kinetic models for OPAH formation chemistry and for achieving emission control.
  • Experimental and modelling study of hydrogen ignition in CO2 bath gas

    Harman-Thomas, James M.; Kashif, Touqeer Anwar; Hughes, Kevin J.; Pourkashanian, Mohamed; Farooq, Aamir (Fuel, Elsevier BV, 2022-11-13) [Article]
    Direct-fired supercritical CO2 power cycles, operating on natural gas or syngas, have been proposed as future energy technologies with 100 % carbon capture at a price competitive with existing fossil fuel technologies. Likewise, blue or green hydrogen may be used for power generation to counter the intermittency of renewable power technologies. In this work, ignition delay times (IDTs) of hydrogen were measured in a high concentration of CO2 bath gas over 1050 – 1300 K and pressures between 20 and 40 bar. Measured datasets were compared with chemical kinetic simulations using AramcoMech 2.0 and the University of Sheffield supercritical CO2 (UoS sCO2 2.0) chemical kinetic mechanisms. The UoS sCO2 2.0 mechanism was recently developed to model IDTs of methane, hydrogen, and syngas in CO2 bath gas. Sensitivity analyses were used to identify important reactions and to illustrate the trends observed among various datasets. The performance of both mechanisms was evaluated quantitatively by comparing the average absolute error between the predicted and experimental IDTs, which showed UoS sCO2 2.0 as the superior mechanism for modelling hydrogen IDTs in CO2 bath gas. The importance of OH time-histories is identified as the most appropriate next step in further validation of the kinetic mechanism.
  • Prediction of the developing detonation regime in a NTC-fuel/air mixture with temperature inhomogeneities under engine conditions

    Luong, Minh Bau; Im, Hong G. (Proceedings of the Combustion Institute, Elsevier BV, 2022-11-12) [Article]
    The developing detonation process resulting in superknock development under realistic engine conditions in the presence of low-temperature chemistry (LTC) is investigated using two-dimensional direct numerical simulations (DNS). A new model is proposed to predict the developing detonation regime under engine conditions. The prediction is validated against the DNS results. Dimethyl ether (DME) exhibiting a typical negative temperature coefficient (NTC) behavior is used as a fuel. In the presence of the NTC regime, the mean distance of dissipation elements of the ignition delay field, lDE, is considered as the characteristic length scale of hot spots to improve the predictive accuracy of the model. The model also accounts for the multi-dimensional effect resulting from the interaction and collision of multiple ignition kernels that is found to promote the onset of detonation and reduce the runup distance of detonation initiation as compared to that of 1D laminar detonation cases. The trasient mixture state is also incorporated in the model development. The model is demonstrated to accurately capture the developing detonation boundary that is consistent with the knock intensity level obtained from the statistical analysis of DNS dataset.
  • Review of life cycle assessments (LCA) for mobility powertrains

    Sarathy, Mani; Nagaraja, Shashank S.; Singh, Eshan; Cenker, Emre; Amer, Amer (Transportation Engineering, Elsevier BV, 2022-11-10) [Article]
    Continued economic growth is accelerating the demand for transport energy in the road, aviation, and marine sectors. The environmental impacts of technologies across these sectors need significant attention if society is to meet its climate change mitigation targets. Current policies focusing entirely on electrification are shown to be insufficient in mitigating environmental impacts. This is because powertrains equipped with combustion engines will still be operational 2040 onwards, especially in the developing countries and currently underdeveloped countries. In addition, the environmental impacts of electrified systems are not negligible, and lifecycle environmental impacts need attention. This paper presents a detailed summary of fuel-powertrain options that society will be dependent on, for next several decades. Environmental impacts using lifecycle assessment (LCA) are presented for various technologies. This review highlights the current inadequacies in application of LCA methods for transport systems, and the need to improve LCA methodologies to drive effective policy and decision making.
  • Evaporation and clustering of ammonia droplets in a hot environment

    Angelilli, Lorenzo; Hernandez Perez, Francisco; Im, Hong G.; Ciottoli, Pietro P.; Valorani, Mauro (Physical Review Fluids, American Physical Society (APS), 2022-11-09) [Article]
    Recent developments in the transition to zero-carbon fuels show that ammonia is a valid candidate for combustion. However, liquid ammonia combustion is difficult to stabilize due to a large latent heat of evaporation, which generates a strong cooling effect that adversely affects the flame stabilization and combustion efficiency. In addition, the slow burning rate of ammonia enhances the undesired production of NOx and N2O. To increase the flame speed, ammonia must be blended with a gaseous fuel having a high burning rate. In this context, a deeper understanding of the droplet dynamics is required to optimize the combustor design. To provide reliable physical insights into diluted ammonia sprays blended with gaseous methane, direct numerical simulations are employed. Three numerical experiments were performed with cold, standard, and hot ambient in nonreactive conditions. The droplet radius and velocity distribution, as well as the mass and heat coupling source terms are compared to study the effects on the evaporation. Since the cooling effect is stronger than the heat convection between the droplet and the environment in each case, ammonia droplets do not experience boiling. On the other hand, the entrainment of dry air into the ammonia-methane mixture moves the saturation level beyond 100% and droplets condense. The aforementioned phenomena are found to strongly affect the droplet evolution. Finally, a three-dimensional Voronoi analysis is performed to characterize the dispersive or clustering behavior of droplets by means of the definition of a clustering index.
  • Quantitative laser-induced fluorescence of NO in ammonia-hydrogen-nitrogen turbulent jet flames at elevated pressure

    Wang, Guoqing; Tang, Hao; Yang, Chaobo; Magnotti, Gaetano; Roberts, William L.; Guiberti, Thibault (Proceedings of the Combustion Institute, Elsevier BV, 2022-11-09) [Article]
    Ammonia is a very promising carbon-free fuel, but its combustion is prone to generate a large amount of harmful nitric oxide (NO). Designing NO reduction strategies for ammonia flames requires computational fluid dynamics and accurate kinetic mechanisms. However, there are currently no available experimental data that can be used to validate models describing chemistry-turbulence interactions in ammonia flames. This study introduces two non-premixed turbulent jet flames that emulate some features of the cracked ammonia combustion at 5 bar, relevant to micro gas turbines. These ammonia-hydrogen-nitrogen jet flames feature well-controlled boundary conditions and are particularly amenable to modeling. A one-dimensional NO laser-induced fluorescence (NO-LIF) method was implemented and combined with 1-D Raman spectroscopy to measure the NO mole fraction quantitatively. To avoid laser absorption by ammonia, excitation in the A2 + − X 2 (0–1) band near 236 nm was chosen instead of the more conventional (0–0) band near 226 nm. Due to the unsteady nature of turbulent jet flames, offline NO-LIF measurements are not useful, and undesirable interferences from Rayleigh scattering and oxygen LIF were instead quantified and removed using the temperature and major species mole fractions measured with Raman spectroscopy. NO quantification algorithms were optimized and validated first with a laminar NH3–H2–N2 counterflow flame, and then applied to the turbulent jet flames. Results show that a large amount of NO (∼1500 ppm) is produced in the NH3–H2–N2 jet flames. Increasing the ammonia cracking ratio from 14% to 28% reduces the NO concentration in laminar and turbulent NH3–H2–N2 flames. Data also shows that turbulence smear effects of differential diffusion. To the best of our knowledge, this is the first available database featuring quantitative measurements of spatially-resolved NO mole fraction in turbulent NH3–H2–N2 flames at a practically-relevant pressure. This unique database can be used in the future to validate turbulent combustion models for such flames.
  • Effects of oxygen partial premixing on soot formation in ethylene counterflow flames with oscillating strain rates

    Zhou, Mengxiang; Xu, Lei; Yan, Fuwu; Chung, Suk Ho; Wang, Yu (Combustion and Flame, Elsevier BV, 2022-11-04) [Article]
    The sooting characteristics of counterflow partially-premixed flames subject to monochromatic oscillation of strain rates were investigated. Strain rate oscillations with well controlled frequencies and amplitudes were imposed on a series of ethylene counterflow partially-premixed flames. Flow fields and soot volume fractions were measured non-intrusively in a spatially and temporally resolved manner. Numerical simulations were also performed to support the experimental observations. The results showed that oxygen partial premixing in the ethylene fuel stream notably increased soot production due to the enhanced temperature and oxidative pyrolysis that led to increased formation of benzene precursors. More importantly, despite these enhancing effects on soot loading, we demonstrated for the first time that partial premixing does not alter the sensitivity of soot formation to strain rate in steady counterflow flames. In addition, when subject to oscillating strain rates, the unsteady responses of partially-premixed flames were still seen to be independent of partial premixing. This indicates that our previous conclusion—the sensitivity of soot formation to strain rate in steady counterflow diffusion non-premixed flames determines its response under unsteady condition—still holds for partially-premixed flames. The results could be useful for the development of advanced flamelet model to quantitatively account for unsteady effects on soot formation in practical turbulent flames that frequently encounters partial premixing.
  • Dimethyl Ether Low-Temperature Catalytic Oxidation Over Rh/Rh/Al2O3 in a Stagnation-Flow Reactor

    Alghamdi, Nawaf M.; Sarathy, Mani (Elsevier BV, 2022-11-03) [Preprint]
    Dimethyl ether (DME) is a promising fuel for use in low-temperature portable hydrogen production, domestic applications, or diesel engines. It burns with less emissions than conventional fuels and has properties similar to LPG in terms of storage and transport, rendering it effective in many strategies for combating climate change. In this study we investigated the partial and total oxidation of DME over 5 wt. % Rh/Al2O3 at low temperatures (215 to 320 ˚C), relevant to portable and domestic energy applications as well as the after-treatment systems of DME-powered engines. We captured the effects of temperature, flow rate, and inlet feed composition on the reactivity. For partial oxidation, we utilized the stagnation-flow reactor geometry to isolate the oxidation zone from the reforming zone. We discuss the reaction order with respect to DME and O2 and provide activation energy values under kinetics control. We also provide data where mass transfer limitations are present to examine the diffusive-convective transport near the catalyst surface, not easily done in three-dimensional environments such as packed beds. The experimental data we provide here pave the way for accurate kinetic modeling of DME partial and total oxidation on Rh/Al2O3, for reactor design and optimization as well as rational catalyst design.
  • Flame detachment of jet fires at windward and leeward sides in crossflow: Experiment and a Damköhler number based model

    Hu, Longhua; Shang, Fengju; Chen, Yuhang; Chung, Suk Ho (Proceedings of the Combustion Institute, Elsevier BV, 2022-11-02) [Article]
    Flame detachment of jet fires are important in practical application such as elevated flares, where flame detachment is used to minimize damage on nozzle and the attendant fire safety problem. In this work, detachment behavior of a jet flame under crossflow was studied, especially focused on the difference in critical crossflow speeds for flame detachments at the windward and leeward sides of the nozzle exit, such problem has not been quantified previously. Experiments were conducted in a wind tunnel using propane as the fuel by adopting jet nozzles with inner diameters of 3, 5, 8, and 10 mm and an outer diameter of 15 mm; together with inner diameters of 13 and 15 mm and an outer diameter of 20 mm. The results showed that the critical crossflow speeds for flame detachments both at the windward and leeward sides decreased as the jet velocity increased. The critical crossflow speed for flame detachment at the windward side was weakly dependent on nozzle diameter, while that at the leeward side increased with the increase in nozzle diameter. A model was proposed to characterize this difference on the basis of a Damköhler number, which is defined as the ratio of the characteristic mixing time to chemical reaction time. A satisfactory correlation was achieved for the difference in critical crossflow speeds for flame detachments at the windward and leeward sides as a function of fuel jet velocity and nozzle dimensions of inner diameter and wall thickness.
  • Microwave-Assisted Solvent Deasphalting of Heavy Fuel Oil: Process Parameters Optimization With RSM and ANN

    Saha, Biswajit; Vedachalam, Sundaramurthy; Paul, Atanu Kumar; Dalai, Ajay K.; Saxena, Saumitra; Roberts, William L.; Dryer, Frederick L. (Elsevier BV, 2022-11-02) [Preprint]
    As petroleum recovery has progressed historically, the portion of heavier crudes and bottom of the barrel residues from the refining process has increased. These crudes are challenging to process, leaving vacuum residues with large fractions of ash and refractory sulfur due to high asphaltene content. Asphaltenes are known to form coke in catalytic upgraders and deactivate refining catalysts. Asphaltenes, which are present in significant amounts in heavy crudes, are the cause of reduction of combustion efficiency, clogging of refinery pipes, and particulate matter emissions. Asphaltenes can be removed from heavy crudes by solvent deasphalting. But the requirement of a high solvent to oil ratio limits its commercial viability. To lower the requirement of solvent, this study investigates deasphalting of heavy fuel oil (HFO) with n-heptane, n-hexane, and n-pentane by microwave-assisted, ultrasound-assisted, and supercritical solvent deasphalting methods. Among the different methods investigated, the microwave-assisted method removed 88 and 80 wt% of asphaltenes from HFO using heptane and hexane, respectively. Microwave irradiation selectively heats asphaltenes in microwave transparent non-polar solvents and increases the degree of collision of asphaltenes for aggregation and thus precipitation. Besides, resins are readily solubilized by the solvent under microwave heating and thus they are unable to act as peptizing agents of asphaltenes. The optimization of process parameters such as solvent to HFO ratio, microwave power, and holding time was investigated for microwave-assisted deasphalting using Central Composite Design (CCD) and artificial neural network (ANN). The optimum removal of asphaltene was observed when the solvent to HFO ratio, microwave power, and holding time were kept at 3, 150W, and 20 min, respectively. Deasphalting also significantly improved the quality of HFO by dropping the viscosity along with the sulfur and nitrogen contents of HFO. The outcomes of this study are significant for the petrochemical industry as potentially improved crude oil processing with lower solvent to HFO ratios can be achieved in a more effective, economical manner using microwave assistance.
  • Effects of Water Vapor Addition on Downstream Interaction in Co/O2 Counterflow Premixed Flames

    Kim, Gyeong Taek; Park, Jeong; Chung, Suk Ho; Yoo, Chun Sang (Elsevier BV, 2022-11-01) [Preprint]
    The effects of H2O on downstream interaction in counterflow premixed CO/O2 flames are investigated by varying the global strain rate (ag) and CO mole fractions (XCO,L, XCO,U) in the lower and upper nozzles, respectively. For interacting premixed CO/O2 flames, the flammable region is very narrow such that the flames cannot be sustained for ag > 11.75 s−1 [[EQUATION]] When 1.0% vol H2O is added to O2/CO2 mixtures, the flammable region is appreciably extended. At low ag, the lean-lean and rich-rich extinction boundaries show strong and weak interaction modes similar to those observed previously in hydrocarbon fuels. The flammable lean-lean and rich-rich regions gradually shrink with the increase of ag. When XCO,U is small for asymmetric lean double flames at low ag, the extinction boundary exhibits weak interaction behavior, where the weaker flame is parasitic to the stronger flame by XCO,L. The stronger flame experiences heat loss to the weaker flame. As ag increases, the reaction cannot be completed due to the reduction in the flow time. The thermal energy loss by incomplete reaction leads to the flame extinction. This effect changes the qualitative nature of extinction boundary as ag increases, resulting in the extinction boundary having only the strong interaction mode having near constant (XCO,L+XCO,U) and bending toward larger XCO,L, and eventually forming an island shape at higher strain rate. The local equilibrium temperature (LET) concept is introduced to explain these flame extinction mechanisms. Local temperature behaviors are well explained by investigating major reaction contributions to heat release rate. In all cases, LET decreases by the effect of preferential diffusion because of the Lewis number of deficient reactant being larger than unity. For asymmetric double flames, conductive heat transfer (CHT) from the stronger to weaker flame reduces the LET of the stronger flame. Flame extinction mechanism can be explained by introducing a loss ratio. For CO/O2 flames, the effects of incomplete reaction as well as preferential diffusion and CHT lead to flame extinction. For (CO/O2+1.0% H2O) flames with the increase of ag, thermal energy loss by incomplete reaction becomes appreciable, as compared with the effects of preferential diffusion and CHT.
  • High-Throughput Experiments and Kinetic Modeling of Oxidative Coupling of Methane, OCM Over La2O3/CeO2 Catalyst.

    Alturkistani, Sultan H.; Wang, Haoyi; Yalamanchi, Kiran; Gautam, Ribhu; Sarathy, Mani (SPE, 2022-10-31) [Conference Paper]
    A reliable dataset covering a parametric space of process conditions is essential for realizing catalyst informatics. A high-throughput screening (HTS) instrument was employed to obtain a parametric dataset to develop a detailed reaction microkinetic model for the oxidative coupling of methane (OCM) over La2O3/CeO2 catalyst. The model was combined with well-validated gas-phase kinetics to describe the interactions between homogeneous and heterogeneous reaction chemistry. Methane and oxygen conversions and selectivities of ethylene, ethane, carbon monoxide, and carbon dioxide were measured experimentally in the temperature range of 500-800 °C, CH4/O2 ratio between 3-13, and pressure between 1 to 10 bar. The proposed reaction network consists of 52 irreversible elementary steps describing catalytic reactions between 11 surface species and 123 reversible steps describing the contribution of gas-phase between 25 species. A packed-bed reactor model was developed based on dimensions of experimental setup and catalyst characterization results to account for homogeneous-heterogeneous interactions. The proposed mechanism was tested and validated over a wide range of operating conditions and showed a reasonable fit with an average difference of less than 5% compared to experimentally measured methane conversion and selectivities of ethylene and ethane. Rate-of-production (ROP) and sensitivity analysis were performed to identify main reaction pathways and highlight the important reactions in the OCM.
  • Evaluation of Miscanthus Gasification and Oxy-Combustion Carbon Dioxide Removal Potential with Carbon Capture Towards Implementation of Bioenergy with Carbon Capture and Storage in England

    Kaiser, David; Sakleshpur, Shashank; Sarathy, Mani; Gautam, Ribhu; Khandavilli, Murali; Arias Gallego, Carolina (SPE, 2022-10-31) [Conference Paper]
    Bioenergy with Carbon Capture and Storage (BECCS) pathways and supply chain designs are researched broadly and implemented for scenarios as of the IEA's (2021) Net Zero by 2050 report. The Committee on Climate Change (2018a, 2018b) has identified Miscanthus as one biomass type to achieve its negative emission goals and aligned one third of 1.2 million hectares under high level and one third of 0.7 million hectares under medium level of ambition (multi-functional land use) for the cultivation of Miscanthus for BECCS in the UK. In this study the input of 39 t/hr of Miscanthus x giganteus biomass as well as Energy technologies institutes (2015) information on projected distributed BECCS installations in the UK for BECCS were considered to bring up different gasifying agent options for H2 generation through Miscanthus Gasification with pre combustion carbon capture and one configuration for oxy-combustion with post combustion carbon capture for highly efficient power generation. Process simulations with Aspen software were conducted to determine power yields and carbon capture rates of optimized bioenergy with carbon capture value chains, sensitivity analysis were executed in order to optimize the configurations. The aim of the study was to observe how highest achievable power generation efficiencies of H2 generation through gasification of Miscanthus x giganetus compare with oxy-combustion power generation efficiency and how the different pathways influence the carbon capture efficiency. The aim was to inform BECCS implementation decisions with optimum possible H2 and power generation yields as well as their respective carbon capture potential. It was found that under oxygen, air and steam as gasifying agents steam is most effective for H2 generation with 3.1 t/hr of H2 produced under a input of 39 t/hr of Miscanthus input, which generates 35,6 MW of power in a simulated H2 turbine. Under simulation assumptions it captures thereby 55,2 t/hr of CO2 with a carbon capture rate of 99%. Oxy-combustion is more efficient than the gasification pathways in regard of power generation, which is 100,4 MW with CO2 capture of 36,6 t/hr with an carbon capture efficiency of 73,8 %. Concluding oxy-combustion is preferred, if highly efficient power generation is wanted and lower carbon capture rate is accepted thereby. When H2 generation is preferred, steam gasification should be chosen as highest efficient gasification pathway. The exact numbers of power generation as well as carbon capture can be used now to estimate UKs overall power generation as well as carbon capture potential of Miscanthus x giganteus cultivation under different land use scenarios considering land use change effects and biodiversity.
  • Lean Stability Limits and Exhaust Emissions of Ammonia-Methane-Air Swirl Flames at Micro Gas Turbine Relevant Pressure

    Avila, Cristian; Wang, Guoqing; Zhu, Xuren; Es-sebbar, Et-touhami; Abdullah, Marwan; Younes, Mourad; Jamal, Aqil; Guiberti, Thibault; Roberts, William L. (American Society of Mechanical Engineers, 2022-10-28) [Conference Paper]
    This study reports on the lean stability limits and exhaust emissions of ammonia-methane-air swirl flames with varied ammonia fuel fractions. A reduced-scale burner was manufactured, inspired by Ansaldo’s micro gas turbine AE-T100 burner, and it was installed inside a high-pressure combustion duct to operate at 4.5 bar. This pressure corresponds to that found at full-load in the actual micro gas turbine’s combustion chamber. The lean stability limits were measured by igniting the flame at an equivalence ratio of ϕ = 0.85 and then progressively decreasing the equivalence ratio until lean blowout. Emissions of CO2, NO, and N2O were recorded for different equivalence ratios and ammonia fractions. Rich flames at an equivalence ratio of ϕ = 1.20 were also considered. Results show that the equivalence ratio at lean blowout increases when the ammonia fraction increases and that all the ammonia fractions tested lead to flames more prone to lean blowout than the pure methane reference flame. The CO2 emissions are monotonically reduced by increasing the ammonia fraction, both for lean and rich flames. The NO emissions exceed many regulations limit regardless of the ammonia fraction for all lean equivalence ratios. N2O emissions are almost negligible, except for very lean equivalence ratios where the N2O mole fraction in the exhaust reaches unacceptably high values. Only rich ammonia-methane-air flames show good NO and N2O performance. Therefore, FTIR analysis was carried out to quantify the amount of the unburnt NH3 in the exhaust for these flames. Results show that unburnt NH3 concentration is invariant, around 200 ppmv, between 0.70 ≤ XNH3 ≤ 0.95. Data reported in this study provide insights for future work on combustors and after-treatment systems towards zero-emissions micro gas turbines.
  • Numerical Investigation on Regenerative Heat-Driven Cryocoolers for Zero Boil-Off Storage of Liquid Hydrogen

    Luo, Jing; Chen, Yanyan; Zhang, Limin; Sankar, Vigneshwaran; Prabhudharwadkar, Deoras Mukund; Saxena, Saumitra; Lacoste, Deanna; Roberts, William L.; Luo, Ercang (American Society of Mechanical Engineers, 2022-10-28) [Conference Paper]
    Aiming at the zero boil-off demand of liquid hydrogen storage tank, this paper uses SAGE software to design and simulate 300 W @ 20 K regenerative heat-driven cryocoolers, verifying the principle feasibility of duplex free-piston Stirling cryocooler and thermoacoustic heat-driven pulse tube cryocooler for the zero boil-off storage of liquid hydrogen. The results show that under the design conditions of mean pressure of 5 MPa, the operating frequency of 50 Hz, the heating temperature of 500°C, and ambient temperature of 30°C, the exergy efficiency of the duplex free-piston Stirling cryocooler can reach 19.4%, while the exergy efficiency of the thermoacoustic heat-driven pulse tube cryocooler is 14.3%. However, because a fixed-parameter harmonic oscillator is used to couple the engine and the cryocooler in the duplex free-piston Stirling cryocooler, it is difficult to achieve multi-condition matching, which makes it very sensitive to changes in operating parameters such as mean pressure and heating temperature. In contrast, the thermoacoustic heat-driven pulse tube cryocooler is completely free of moving parts and has excellent adaptability to the operating conditions. Therefore, the thermoacoustic heat-driven pulse tube cryocooler may be a promising solution in the application field of zero boil-off storage of liquid hydrogen.
  • A generalized partially stirred reactor model for turbulent closure

    Quadarella, Erica; Péquin, Arthur; Stagni, Alessandro; Parente, Alessandro; Faravelli, Tiziano; Im, Hong G. (Proceedings of the Combustion Institute, Elsevier BV, 2022-10-26) [Article]
    A generalized Partially-Stirred Reactor (PaSR) model is presented in this work based on the inclusion of multiple chemical times. The PaSR model has shown promising results at modelling turbulence-chemistry interaction in Large-Eddy Simulations (LES) and Reynolds-Averaged Navier-Stokes (RANS), providing an extension of the well-known Eddy Dissipation Concept (EDC). PaSR model divides the computational domain into reactive and non-reactive parts. The factor defining this partition is expressed as a function of the system characteristic chemical and mixing times. However, the estimation of these factors, particularly the chemical one, is often oversimplified. The approach proposed in this study seeks to include in the PaSR model the whole set of chemical times involved in the reactive system. Besides, the concept of fine structures, first introduced in the EDC and often adopted also in the PaSR model to characterize the evolution of chemistry in the reactive part of the fluid, is here abandoned in favour of direct manipulation of species production rates. The mean source term is formulated according to the new generalized model through a modal decomposition of the Jacobian matrix. The method is validated a priori with DNS data of a syngas non-premixed jet flame, whose filtered data represent the validation benchmark. A good agreement is found between the new PaSR model and the filtered data for all species at different filter widths. Comparison with the single time scale based model clearly shows the limitations of the old standard approach and the necessity of including the whole spectrum of chemical times for a more comprehensive description of turbulence-chemistry interaction. A thorough analysis with the time scale participation index reveals the complexity of reaction rates contributions to the development of a specific time scale, underlying the importance of developing a model able to inherit all kinetic pathways in the turbulent closure.
  • Investigation of cyclohexene thermal decomposition and cyclohexene + OH reactions

    Liu, Dapeng; Farooq, Aamir (Combustion and Flame, Elsevier BV, 2022-10-25) [Article]
    Cyclohexene is a common intermediate in the oxidation of cyclohexane and alkyl-substituted cyclohexanes. It also plays an important role in the formation of polycyclic aromatic hydrocarbons (PAHs) by providing a route for the first aromatic ring. Two types of reactions are highly important in cyclohexene kinetics, i.e., H-abstraction by OH radicals and cyclohexene thermal decomposition. In this work, we have investigated these two reactions experimentally by employing sensitive UV diagnostics of OH radicals and 1,3-butadiene. We conducted rate coefficient measurements of cyclohexene + OH reaction (k1) over 933–1259 K and 1–4 bar using a narrow-linewidth UV absorption diagnostic of OH radicals near 306.67 nm. The carefully designed test conditions minimized the influence of secondary reactions such as cyclohexene thermal decomposition. Our determined OH + cyclohexene high-temperature rate coefficients show a positive temperature dependence and a negligible pressure dependence. Our rate values may be fitted with a two-parameter Arrhenius expression (units of cm3molecule−1s−1): k1 = 1.28 × 10−10 e( −1225/T ) We also investigated channel-specific competition in OH + cyclohexene reaction. H-abstraction by OH from allylic (kc), alkylic (kb), and vinylic (ka) Csingle bondH bonds contributes roughly 60%, 35%, and 5% over our temperature range. Our determined kc agrees excellently with literature rate coefficient of H-abstraction from the allylic site of 1-butene. We studied cyclohexene thermal decomposition reaction (k2) by tracing the product 1,3-butadiene near 212.5 nm over 1092–1361 K and 0.82–1 bar. Compared to literature works, our highly sensitive UV diagnostic of 1,3-butadiene enabled time-resolved measurements with low cyclohexene concentration, which guaranteed nearly isothermal conditions despite reaction endothermcity. Our measured rate coefficients may be expressed as (unit of s−1): k2 = 3.68 × 1014e( −31,562/T ) We implemented our determined rate coefficients (k1 and k2) in a literature cyclohexene kinetic model and also updated the 1,3-butadiene sub-mechanism. The updated model shows improved predictions of measured ignition delay times of cyclohexene.
  • Probing the influence of hydrogen cyanide on PAH chemistry

    Liu, Peng; Chen, Bingjie; Bennett, Anthony; Pitsch, Heinz; Roberts, William L. (Proceedings of the Combustion Institute, Elsevier BV, 2022-10-23) [Article]
    Hydrogen cyanide (HCN) is an important and highly toxic intermediate in coal, nitrogen-rich biomass, and ammonia combustion. It may play an important role in polycyclic aromatic hydrocarbons (PAH) and soot reduction, but the influence of HCN on PAH chemistry is unclear due to the lack of experimental data. With this motivation, the experimental and numerical investigations were performed in this work. Key intermediates were detected, identified, and quantified by the combination of Linear Trap Quadropole (LTQ) Velos Orbitrap mass spectrometer and gas chromatography-mass spectrometer in a jet-stirred reactor fueled by C2H2/HCN/N2 in the temperature range of 800-1200 K. The results reveal that the formation of benzene increases with the addition of HCN, but the PAH formation decreases. PAH reduction can be attributed to the formation of N-containing PAH (NPAH) via HCN-PAH interaction reactions, which were investigated by quantum chemistry and the Rice–Ramsperger–Kassel–Marcus theory with solving the master equation (RRKM-ME). Reaction rate comparison suggests that the HCN addition pathways to NPAH compete with the C2H2 addition pathways to PAH. The product yields in the systems of 1-naphthyl radical + C2H2 and 1-naphthyl radical + HCN were evaluated (T=800-2500 K and p=0.1-100 atm). The results indicate that the increasement of a new aromatic ring via cyclization is difficult in the 1-naphthyl radical + HCN system due to the high energy barrier, and the growth of larger NPAH is limited since the saturated N atoms in the heterocycle rings and Ctriple bondN functional group inhibit further carbon addition.

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