H2 Mole Fraction Research Articles

Thermodynamic analysis of using CO2 as a co-gasification agent in supercritical water gasification of glycerol

Using CO2 as a co-gasification agent in supercritical water gasification (SCWG) of glycerol is one of the promising approaches for syngas production. In this study, a non-stoichiometric two-phase model (i.e., supercritical and solid phase) is used for thermodynamic analysis of supercritical H2O-CO2 co-gasification (SCWCG) of glycerol. The Gibbs free energy of the entire system is minimized to calculate the equilibrium compositions of gaseous and solid products. The chemical potentials of gaseous products are accurately predicted by using the PR-BM equation of state. The validity of the established model is initially confirmed through its fitting with experimental data. Subsequently, the model is used to investigate the effects of various single and combined operating conditions on product compositions. The findings indicate that temperature, CO2 addition and glycerol concentration have significant influence on the process performance, whereas the pressure has a negligible effect. In addition, the theoretical maximum H2 molar fraction of 68.15% are obtained. Furthermore, the reaction heat duty and high heating value of gaseous products are analyzed. When the glycerol concentration is low, the SCWCG process turns out to be endothermic and can be energetically self-sufficient by adding a small amount of O2 at the expense of reducing the heating value of gaseous products. The study provides valuable insights on designing an optimal SCWCG process for syngas production from glycerol.

Utilization of mill rejects with biomass pellets in the existing coal power plants- a novel approach towards sustainability & fuel security

ABSTRACT The present work is focused on the possible utilization of mill rejects blended with the biomass non-torrefied pellets (made of paddy straw, dry grasses and fallen sal leaves) in the existing tangential coal-fired boiler through the gasification process. The physio-chemical analysis and combustion behavior study of the materials is done based on thermo-gravimetric analysis (TGA) and differential thermal analysis (DTA). The result shows that the gross calorific value (GCV) of the developed bio-mass pellet is about 3900–4300 kCal/kg with the mass percentage of fixed carbon (FC) and volatile matter (VM) of about 15% and 70%, respectively. From the TGA and DTG study, it is observed that about 7% weight loss was found in all pellets at a temperature range of about 85–110°C but for the mill rejects, it is about 5–6%. At the peak temperature, the maximum weight loss of pellets and mill rejects is about 0.90 gm/min and 0.16 gm/min, respectively. The combined use of mill rejects and pellets (1:1) in the updraft gasifier generates combustible syngas with mole fractions of CO and H2 of about 0.29 and 0.30, respectively with a lower heating value of about 159 kJ/mole. The direct use of generated combustible syngas into the coal-fired boiler of a 500 MW power plant increases the net energetic and exergetic performance by about 0.09 and 0.07% points, respectively and resulting in a reduction of annual CO2 emissions by about 9452 tons. Cost-benefit analysis is used to determine the feasibility of the system.

An energy-efficient and cleaner production of hydrogen by steam reforming of glycerol using Aspen Plus

Hydrogen (H2) is produced in an efficient and sustainable manner using steam reforming of glycerol in this work. Two models are used to provide anticipation about how efficiently H2 can be produced: Model 1, a stoichiometric model, and Model 2, a thermodynamic model. Model 1 helps determine the water/glycerol molar input ratio and reaction pathway. The thermodynamic feasibility of Model 1 is examined using Model 2, which then determines the optimum conditions for maximal H2 generation. Model 2 produced 1275 kmol/h of H2 with an H2 mole fraction of 0.49 at 658 °C and 1 atm of pressure. The reactor's net heat load is greatly reduced through heat integration in this study. The energy required is reduced by 29.5% as compared to when no heat integration is used. The process of separating and eliminating CO2 is accomplished by employing the principles of mass and energy balance in the Aspen Plus.

Solar-driven biomass steam gasification by new concept of solar particles heat carrier with CPFD simulation

Solar-driven biomass gasification is a promising solar thermochemical technology to produce green H2-enriched syngas, and contribute to solar energy upgrade and efficient conversion. Whereas, the continuous and stable operation of solar-driven biomass gasification is challenging due to the non-uniform concentration of solar flux and its fluctuating nature. In this regard, a novel concept of solar-driven gasification is proposed, using solar-heated particles as the heat carrier, and biomass steam gasification in a fluidized bed reactor is numerically investigated based on the computational particle fluid dynamics (CPFD) model. The complex distributions of gas and particle phases, as well as the effects of operation parameters on syngas species are comprehensively analyzed. Results indicate that the biomass material and solar-heated particles can be well-mixed through continuous feeding, and the internal heat transfer and gasification kinetics are enhanced. The mole fraction of H2 and CO in the produced syngas reach to 50.86% and 18.97% under the benchmark operating condition. By using the high-temperature quartz sand as the solar particles heat carrier, the long-term and continuous operation of the endothermic gasification process can be achieved. The newly developed solar-driven biomass gasification method provides a promising approach to mitigate the impact of the fluctuating solar irradiation and enhance the operational flexibility of gasification process.

High Dimensional Model Representation Approach for Prediction and Optimization of the Supercritical Water Gasification System Coupled with Photothermal Energy Storage

Supercritical water gasification (SCWG) coupled with solar energy systems is a new biomass gasification technology developed in recent decades. However, conventional solar-powered biomass gasification technology has intermittent operation issues and involves multi-variable characteristics, strong coupling, and nonlinearity. To solve the above problems, firstly, a solar-driven biomass supercritical water gasification technology combined with a molten salt energy storage system is proposed in this paper. This system effectively overcomes the intermittent problem of solar energy and provides a new method for the carbon-neutral process of hydrogen production. Secondly, the high dimensional model representation (HDMR) approach, as a surrogate model, was used to predict the production and lower heating value of syngas developed in Aspen Plus, which were validated using experimental data obtained from the literature. The ultimate analysis of biomass, temperature, pressure, and biomass-to-water ratio (BWR) were selected as input variables for the model. The non-dominated sorted genetic algorithm II (NSGA II) was considered to maximize the gasification yield of H2 and the LHV of syngas in the SCWG process for five different types of biomass. Firstly, the results showed that HDMR models demonstrated high performance in predicting the mole fraction of H2, CH4, CO, CO2, gasification yield of H2, and lower heating value (LHV) with R2 of 0.995, 0.996, 0.997, 0.996, 0.999, and 0.995, respectively. Secondly, temperature and BWR were found to have significant effects on SCWG compared to pressure. Finally, the multi-objective optimization results for five different types of biomass are discussed in this paper. Therefore, these operating parameters can provide an optimal solution for increasing the economics and characteristics of syngas, thus keeping the process energy efficient.

Effect of hydrogen addition on the laminar burning velocity and the flame stability of n-dodecane reacting with air at elevated pressures

The effects of hydrogen addition on the premixed laminar burning characteristics of n-dodecane reacting in air were experimentally investigated using the freely expanding spherical flame method. The initial operating conditions were temperature = 425 K, pressure = 1 ˗ 4 bar, equivalence ratio = 0.8–1.4, and mole fraction of H2 in n-dodecane-H2 mixture = 0–40%. Nonlinear stretch extrapolation schemes were implemented to minimize the uncertainty in the estimation of unstretched flame speed as the binary fuel mixtures of n-dodecane and H2 had non-unity Lewis numbers. All experimental operating conditions were simulated in CHEMKIN using a planar flame model with three different reaction mechanisms. The statistical uncertainty estimated for the LBV was below ±6.93%. The LBV increased by three/two times at off–stoichiometric/stoichiometric mixtures as the mole fraction of hydrogen (XH2) increased from 0 to 40% in n-dodecane. The rich mixtures of n-dodecane-air (ϕ = 1.4) were unstable to thermo-diffusive effects due to the lower mass diffusivity of n-dodecane and Le < 1, in contrast, H2 blending to the n-dodecane by keeping the other parameters being same transformed it from an unstable to a stable one. The effective Lewis number and Markstein length decreased with an increasing XH2 in n-dodecane, indicating that the mixtures were stable to thermo-diffusive effects, but with further increase in XH2, there may be a transition in flame stability. H2 addition resulted in an earlier onset of hydrodynamic instability due to a reduction in the flame thickness. In addition, sensitivity analysis was performed to identify the key reactions responsible for the enhanced reactivity associated with H2 addition. The reaction pathway and emission analysis showed a significant reduction in CO2 and CO emissions. Finally, an increase of initial pressure decreased the LBV for all investigated mixtures.

Gas-heat-electricity poly-generation system based on solar-driven supercritical water gasification of waste plastics

Energy shortage and environmental pollution make people aware of the need to recycle waste resources and develop environment-friendly energy. The massive plastic waste and clean solar energy provide new solutions to energy and environment issues. In this study, a solar-driven gas-heat-electricity poly-generation system based on supercritical water gasification of plastics was established. Plastic waste and solar energy were converted into electricity, heat and hydrogen-rich gas. A detailed analysis of mass, energy and exergy flow of the system was analyzed under typical operating conditions. The effects of temperature, pressure, plastics to water ratio, etc., on gasification performance, system efficiency, and energy output distribution were investigated. The results showed that the system exergy loss mainly occurred in cooler, reactor and heat exchanger, and their loss account for more than 85% of the total loss. The increase in temperature significantly increased total gas production and H2 molar fraction. At 800 °C, the total gas production reached 77.50 kg/h and the H2 molar fraction was 65.78%. However, increasing temperature had a negative impact on system efficiency. The effect of pressure on the gasification reaction and system efficiency was not significant. Increasing plastics to water ratio was beneficial in improving system efficiency, while each gas production showed varying degrees of growth. However, H2 yield and molar fraction decreased dramatically. The increase of turbine feed water led to a slight decrease of energy efficiency, while the exergy efficiency remained basically unchanged. These can provide theoretical guidance for system optimization and promote the industrial application of the technology.

Numerical analysis of plasma gasification of hazardous waste using Aspen Plus

The COVID-19 pandemic raised the problem of dealing with the hazardous wastes generated. The World Health Organization (OMS) recommends the treatment of these wastes at high temperatures in order to neutralize their negative impact. For this reason, the main objective of this work is the development and analysis of a sustainable way to treat hazardous wastes generated by the COVID-19 pandemic. Thus, to achieve that goal, this paper presents an improved computational model that replicates a high-temperature thermal treatment system for COVID-19 wastes using plasma gasification in Aspen Plus V12.2. The distinctive aspect of the present plasma gasification model is the inclusion of an extra Gibbs reactor in order to enhance the calorific value of the syngas. The model validation results show an increase in the CO and CH4 molar fractions and a decrease of the H2 and CO2 molar fractions, which allows to increase the calorific value of the syngas from 4.97 to 5.19 MJ/m3. The most common types of hazardous waste generated during the pandemic were determined to be masks and syringes. COVID-19 waste from Turkey, discarded masks from Indonesia, Korea, and Lithuania, and Chinese syringes were used as feedstock into the computational model. The results suggest that the hazardous waste that allows for higher hydrogen molar fractions is Korean masks. On the other hand, the highest carbon monoxide molar fractions are obtained with medical waste from Turkey, while the highest molar fractions of methane are obtained with medical waste from Lithuania. A conclusion could be drawn that the lowest syngas calorific value is obtained with medical wastes from Turkey, while the highest syngas calorific value is obtained with medical wastes from Korea.

Study on the ignition characteristics of CH4/H2/air mixtures in a micro flow reactor with a controlled temperature profile

The ignition characteristics of stoichiometric CH4/H2/air mixtures with different H2 dilution ratios were studied by using a micro flow reactor (MFR) with a controlled temperature profile. The weak flame positions of the mixtures were determined by the luminosity of CH∗, and the ignition temperatures (defined as the corresponding wall temperatures) were obtained. The ignition temperature of the mixture is smoothly decreased with a small amount of H2 addition, while the decrease of the ignition temperature is more rapid with increasing H2 dilution. One-dimensional computation with a detailed reaction mechanism was also conducted to study the ignition characteristics of the CH4/H2/air mixtures. The computational ignition temperature which is defined as the wall temperature at the peak of the heat release rate (HRR) peak is also obtained. Both the experimental and computational results show that the weak flame shifts to the lower temperature region with an increase of the H2 mole fraction. And the non-linear decrease of the ignition temperature of the CH4/H2/air weak flame versus increasing H2 dilution was also well reproduced by the numerical computation. The flame structures, primary exothermic and endothermic reactions of the CH4/H2/air weak flames were studied in detail. The importance of H and OH in the ignition process of the mixtures was analyzed, and the kinetic effects of H2 on the ignition of the mixture in the intermediate-temperature region (1000–1200 K) and high-temperature region (1200–1300 K) were elucidated. The analysis of the computational results reveals that the OH production routes in the intermediate-temperature region are substantially changed with different H2 dilutions, while the OH production routes at the peak of the heat release rate are not obviously influenced. In the end, the primary mechanism for the non-linear effects of H2 addition on the ignition temperatures of the CH4/H2 mixtures was clarified.

Enhancement of formic acid production from carbon dioxide hydrogenation using metal-organic frameworks: Monte Carlo simulation study

Formic acid production from CO2 allows the reduction of carbon dioxide emissions while synthesizing a product with a wide range of applications. CO2 hydrogenation is challenging due to the cost of transition metal catalysts and the toxicity of the transition elements. In this work, the thermodynamic confinement effects of the metal–organic framework UiO-66 on the CO2 hydrogenation to formic acid were studied by force field-based molecular simulations. The confinement effects of UiO-66 and the metal–organic frameworks Cu-BTC, and IRMOF-1 were compared, to assess the impact of different pore size and metal centers on the production of HCOOH. Monte Carlo simulations in the grand-canonical ensemble were performed in the frameworks, using gas phase mole fractions of CO2, H2, and HCOOH at chemical equilibrium, obtained from Continuous Fractional Component Monte Carlo simulations in the Reaction Ensemble. The adsorption isobars of the components in metal–organic frameworks were computed at 298.15 – 800 K, 1 – 60 bar. The enhancement of HCOOH production due to preferential adsorption of HCOOH in metal–organic frameworks was calculated for all studied conditions. UiO-66, Cu-BTC, and IRMOF-1 affect CO2 hydrogenation reaction, shifting the thermodynamical equilibrium toward HCOOH formation. The prevailing factor is the type of metal center in the metal–organic framework. The confinement effect of Cu-BTC turns out to exceed the enhancement caused by UiO-66, and IRMOF-1. The resulting mole fraction of HCOOH increased by ca. 2000 times compared to the gas phase at 298.15 K, 60 bar. Cu-BTC can be considered as an alternative to improve the production of HCOOH due to elimination of the high-cost temperature elevation, cost reduction of downstream processing methods, and comparable final concentration of HCOOH to the reported concentrations of formate obtained using transition metal catalysts.

Hydrogen production from sorption enhanced steam reforming of ethanol using bifunctional Ni and Ca-based catalysts doped with Mg and Al

This work evaluated the performance of nickel-based catalysts supported on CaO and CaO–MgO–Al2O3 in the sorption enhanced steam reforming of ethanol (SESRE) aiming the production of high purity H2. The catalysts were prepared by sol-gel method and characterized by different methods: Temperature programmed reduction (TPR), X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) with chemical element mapping, N2 physisorption and CO2 capture capacity determined by thermogravimetric analysis (TGA). XRD analysis showed that the predominant phases were CaO, MgO, CaCO3, Ca(OH)2 and NiO in the calcined samples and Ni0 in the reduced and passivated samples. TPR profiles indicated that all catalysts presented a high degree of reduction (Ni/CaMgAl-68 > Ni/CaMgAl-79 > Ni/Ca), although Ni/CaMgAl-X samples presented high reduction temperatures indicating the formation of NiAl2O4. The addition of MgO and Al2O3 to CaO was very beneficial since the deactivation coefficients, calculated by the TGA data modeling, decreased by a factor of 3.8 for Ni/CaMgAl-79 and by a factor of 4.3 for Ni/CaMgAl-68 when compared to the Ni/Ca catalyst. The catalytic tests in the SESRE showed that Ni/CaMgAl-79 catalyst had the best performance since it had the longest high purity hydrogen production time. In the pre-breakthrough period, the H2 mole fractions were close to 90% for all samples during all reaction cycles. After the reaction-regeneration cycles, the average crystallite size of CaO estimated by XRD increased around 38, 6 and 35% for Ni/Ca, Ni/CaMgAl-79 and Ni/CaMgAl-68, respectively. Thus, adding a dopant to the sorbent material proved to be an effective strategy to obtain a more stable catalyst capable to produce hydrogen of high purity.

Green route for biomethane and hydrogen production via integration of biogas upgrading using pressure swing adsorption and steam-methane reforming process

Biogas is a cornerstone within a clean and sustainable energy portfolio, while hydrogen production from biogas is a key enabler for methane conversion and carbon dioxide valorization for greenhouse gases emissions mitigation. In this work, pressure swing adsorption (PSA) configuration of two vessels-four adsorption beds connected in series was tested for upgrading low-grade biogas (50% CH4 and 50% CO2). To resolve CH4 spillage issue, steam-methane reforming (SMR) plant was proposed as a green route for converting CH4 and portion of CO2 in the waste stream of PSA system into hydrogen. The integrated system (PSA-SMR) was performed on two stages i.e., PSA runs were conducted experimentally in lab while SMR system was simulated using Aspen Hysys software under operating conditions from a real plant. Response surface methodology was applied into Design Expert software to optimize the effects of system pressure, CH4 concentration, and biogas/steam flowrate ratio on H2, CO2, H2O, and CO molar fractions in the product. The results revealed that four adsorption beds in serial configuration successfully recorded ultrapure CH4 of 99.9% and recovery of 84.9% along with average CO2 content of 60% in the waste stream. For SMR system, produced syngas comprised of 42.2% H2, 28.46% CO2, 13.84% N2, 8.88% H2O, and 6.66% CO with 100% conversion of the CH4 and about 52.56% conversion of the CO2. The optimum conditions that achieved the highest H2 content of 51% from SMR were system pressure below 32 bar, methane content in feed stream ≥61%, and biogas/steam ratio in the range of 0.41–0.66 to record H2.

Application and comparison of multiple machine learning models for the prediction of the laminar burning velocity for CH3OH/H2/air mixtures

Recent investigations have indicated that co-firing CH3OH with H2 is a promising approach to develop a carbon-neutral energy system. However, accurate measurements of laminar burning velocities over a wide range of equivalence ratios, H2 mole fractions, pressures and temperatures are complicated and may not available. Hence, this research deeply investigates the application of several machine learning models in predicting the laminar burning velocities of CH3OH/H2 blended fuels. Results denoted that Random Forest Regressor is the most persuasive model based on a thorough comparison, as indicated by the correlation coefficient of 0.99707.

Simulation of Sorption-Enhanced Steam Methane Reforming over Ni-Based Catalyst in a Pressurized Dual Fluidized Bed Reactor

Steam methane reforming is a major method of hydrogen production. However, this method usually suffers from low energy efficiency and high carbon-emission intensity. To solve this issue, a novel steam-methane-reforming process over a Ni-based catalyst in a pressurized dual fluidized bed reactor is proposed in this work. A three-dimensional computational fluid dynamics (CFD) model for the complex physicochemical process was built to study the reforming characteristics. The model was first validated against the reported data in terms of hydrodynamics and reaction kinetics. Next, the performance of the proposed methane-steam-reforming process was predicted. It was found that the methane-conversion ratio was close to 100%. The mole fraction of H2 in the dry-yield syngas reached 98.8%, the cold gas efficiency reached 98.5%, and the carbon-capture rate reached 96.4%. It is believed that the proposed method can be used for methane reforming with high efficiency and low carbon intensity.

High-Dimensional Model Representation-Based Surrogate Model for Optimization and Prediction of Biomass Gasification Process

Biomass gasification process has been predicted and optimized based on process temperature, pressure, and gasifying agent ratios by integrating Aspen Plus simulation with the high-dimensional model representation (HDMR) method. Results show that temperature and biomass to air ratio (BMR) have significant effects on gasification process. HDMR models demonstrated high performance in predicting H2, net heat (NH), higher heating value (HHV), and lower heating value (LHV) with coefficients of determination 0.96, 0.97, 0.99, and 0.99, respectively. HDMR-based single-objective optimization has maximum outputs for H2, HHV, and LHV (0.369 of mole fractions, 340 kJ/mol, and 305 kJ/mol, respectively) but NH would be negative at these conditions, which indicates that process is not energy-efficient. The optimal solution was determined by the multiobjective which produced 0.24 mole fraction of H2, 158.17 kJ/mol of HHV, 142.48 kJ/mol of LHV, and 442.37 kJ/s NH at 765°C, 0.59 BMR, and 1 bar. Therefore, these parameters can provide an optimal solution for increasing gasification yield, keeping process energy-efficient.

Valorization of Sugarcane Bagasse for Hydrogen-Rich Gas Production using Thermodynamic Modeling Approach

Hydrothermal gasification also known as supercritical water gasification (SWG) has been considered a promising approach for converting wet biomass such as sugarcane bagasse into high-quality syngas. This study presents the thermodynamic modeling of the hydrothermal gasification of sugarcane bagasse using Aspen Plus. The effects of process parameters on the composition and yield of product gases were also investigated. It was found that the effect of temperature and biomass concentration were significant in the production of hydrogen-rich gas, while less impact was observed with pressure. The hydrogen gas (H2) produced with the highest mole fraction (56.70 mol%) and yield (103.26 kmol/kg) was obtained at 750°C and low biomass concentration of 10 wt%, while the lowest yield (1.52 kmol/kg) and mole fraction (2.45 mol%) of H2 were obtained at 450°C and high biomass concentration of 50 wt%. Findings from this study also showed that the highest net calorific value (17.55MJ/kg) was reached at 450˚C and 50 wt% of biomass concentration. This study would help to consolidate research on hydrothermal gasification of sugarcane bagasse and optimization of experimental processes and also serve as an important benchmark in the utilization of biomass as a clean energy source for future projects.

Continuous hydrate-based CO2 separation from H2 + CO2 gas mixture using cyclopentane as co-guest

CO2 separation from H2 + CO2 gas mixture is a necessary technology for a stable supply of blue hydrogen. Among CO2 separation technology, hydrate-based CO2 separation is an environmentally and economically superior technology. Cyclopentane (CP) was selected as guest compound to enable hydrate-based CO2 separation under thermodynamic conditions closer to normal pressure and temperature. Hydrate-based continuous CO2 separation experiments are conducted in the H2 + CO2 + CP + H2O system at 284 K. The results of these experiments showed that mole fraction of H2 in the gas phase reached 0.9 at the steady state in the H2 + CO2 + CP + H2O system after the CO2 was extracted via hydrate formation. At the same time, more H2 was incorporated into the mixed hydrate during this process as the concentration of H2 in the gas phase increases post hydrate formation. Therefore, it was demonstrated that hydrate-based CO2 separation was successful in H2 + CO2 + CP + H2O system at 284 K but there is also increase of energy costs caused by H2 loss. The PXRD results identified H2 + CO2 + CP hydrate as structure II hydrate. Although H2 + CO2 + Tetrahydropyran (THP) hydrate also forms structure II hydrate, more H2 is encapsulated in the H2 + CO2 + CP mixed hydrate than in the H2 + CO2 + THP mixed hydrate. This difference may be caused by the difference in the lattice constants of CP and THP hydrates, which arises from the difference in molecular size of CP and THP, from the differences between the large guest–host interactions, and from differences arising from hydrogen bonding of THP with the host water framework which can decrease the H2 holding capacity. In this study, it was revealed that the molecular diameter of the guest molecules need also to be considered in addition to the phase equilibrium conditions of the hydrates when selecting guest compound of hydrates.

Techno-economic analysis for hydrogen-burning power plant with onsite hydrogen production unit based on methane catalytic decomposition

Hydrogen (H2) is the fuel of the future since it only produces water as the combustion product. This paper proposes a rational design of a hydrogen-burning power plant integrated with an onsite hydrogen production unit. The H2 production from natural gas is achieved by converting the CH4 to gaseous H2 and solid carbon product based on CH4 catalytic decomposition. Fe-based catalyst is suggested for the onsite production of H2 because it results in the lowest net-levelized cost of electricity (LCOE) compared to using Ni-based and activated carbon catalysts. The mole fraction of H2 in the fuel increases with the increase of natural gas bypass ratio and CH4 conversion rate from 10 to 100%, and the maximum mole fraction of H2 reaches 93.3%. The studied hydrogen-burning power plant can reduce the CO2 emission by 80.2% compared to direct power generation from natural gas at the cost of 44% less power due to the lower heating value of H2 compared to CH4. It is noted that selling the carbon product becomes an important income source to subsidies the plant cost. However, the increase in the catalyst price and the decrease in the carbon selling price may hinder the profitability of the power plant. Finally, a set of natural gas bypass ratios and CH4 conversion rates is recommended to ensure the power plant produces at least 80% of the electricity compared to the direct power generation from natural gas; it also achieves a more competitive net-LCOE even under the catalyst cost and carbon selling price varying to 300 and 40%, respectively.

An experimental and kinetic modeling study on NH3/air, NH3/H2/air, NH3/CO/air, and NH3/CH4/air premixed laminar flames at elevated temperature

Ammonia combustion helps to realize the off-site use of renewable energy. To understand the combustion characteristics of NH3, laminar burning velocities (SL) of NH3/air, NH3/H2/air, NH3/CO/air, and NH3/CH4/air with various mole fractions of H2, CO, and CH4 in fuel (α), equivalence ratios (ϕ = 0.7∼1.4) at elevated initial temperature (Ti = 298∼423 K) were measured in a constant-volume combustion vessel. Based on updated chemistry of NH2, NNH, N2H2, and NOx, a detailed kinetic model with 169 species and 1268 elementary reactions was developed and validated against the SL, NOx emissions, NH3/NO interaction, and ignition delay times measured by this work and literature. The experimental results show that SL of NH3/H2/air, NH3/CO/air, and NH3/CH4/air flames have different dependence on α. It is nearly exponential for NH3/H2/air flame, non-linear for NH3/CO/air flame, and nearly linear for NH3/CH4/air flame. NH3/air flame has the strongest temperature dependence, followed by NH3/H2/air, NH3/CH4/air, and NH3/CO/air flames. The updated NH3 chemistry, especially NNH chemistry (NNH = N2 + H, N2H2 + H = NNH + H2, and N2H2 + O = NNH + OH), enables the present model obtain better temperature dependence of NH3/air flame. Kinetic modeling analysis shows that the production and reduction of NOx are preferred by the lean and stoichiometric flames of NH3/H2/air, NH3/CO/air, and NH3/CH4/air because of the increased concentrations of NHi and H/O/OH radicals. In the lean flames of NH3/H2/air, NH3/CO/air, and NH3/CH4/air, H2, CO, and CH4 can promote the conversion of NH2 to NO, and relatively inhibit the NO reduction by NH2, resulting in the increase in NO emissions. In the rich flames of NH3/H2/air and NH3/CO/air, NH2 and NH can produce N2H2 and N2H3 through reactions of NH2 + NH = N2H2 +H and NH2 + NH = N2H3, which dominate the promotion and inhibition of laminar flame propagation, respectively.

Cellular structures of laminar lean premixed H2/CH4/air polyhedral flames

Fundamental studies on the effects of differential diffusion of hydrogen (H2) on flame structure are motivated as the high diffusivity of H2 presents challenges for the modeling and optimization of combustion systems. Polyhedral Bunsen flames are examples of cellular flames mainly induced by the thermal-diffusive and hydrodynamic instabilities, which are characterized by periodic positively curved troughs and negatively curved cusps. Stationary laminar premixed fuel-lean H2/CH4/air polyhedral flames, with 50%, 68% and 79% H2 (by volume) and Lewis number (Le) less than unity, are investigated in this study. The internal scalar structures of cellular troughs and cusps in target flames are measured with a high-spatial-resolution 1D Raman/Rayleigh scattering system, combined with planar laser-induced fluorescence of hydroxyl radicals (OH-PLIF) and chemiluminescence imaging measurements to quantify the cell number and local flame curvature. The performance of the 1D Raman/Rayleigh imaging system is first assessed by comparing measurements of temperature and major species in a laminar premixed counterflow H2/CH4/air twin flame with a corresponding simulation. The results reveal significant combined effects of differential diffusion and curvature on flame structures with differences between trough and cusp regions in the measured mole fractions, equivalence ratio, temperature, and C/H-atom ratio. The positively curved troughs have significantly higher H2 mole fraction compared to the negatively curved cusps, due to the respective focusing/defocusing effect of curvature on highly diffusive H2. Consequently, the local equivalence ratio and temperature in trough regions are higher than those of cusps. With the increase of H2 content in the reactant mixture, the scalar differences between trough and cusp regions are enlarged due to the enhanced effects of curvature and differential diffusion. Near-vertical initial trajectories in H2 mole fraction, equivalence ratio, and C/H-atom ratio plotted against temperature showed that differential diffusion of H2 alters the species mole fractions in the cold reactants (≤ 350 K).