Numerical simulation of the synergistic effects of unwanted combustion enhancement by the C3H2F3Br and C2HF5 blends

Owing to the restriction of using Halon 1301 (CF₃Br) for fire suppression, several alternatives to Halon 1301 have been developed, including C₂HF₅ (HFC-125), C₃H₂F₃Br (2-BTP), and C₆F₁₂O (Novec1230). However, in the Federal Aviation Administration (FAA) Aerosol Can Explosion Test (FAA-ACET), it was found that these alternatives did not suppress lean flames at sub-inert concentrations, but promoted combustion, eventually leading to overpressure. Therefore, they have not been successfully applied in aircraft cargo compartments. Herein, different blend ratios of C₃H₂F₃Br and C₂HF₅ were used to explore their inhibitory effects on combustion enhancement under lean combustion conditions. A chemical kinetic model was developed and validated using a one-dimensional free-propagation flame simulator. The laminar burning velocity predicted by the model was consistent with the experimental results. The adiabatic flame temperature and overall reaction rate were determined using thermodynamic equilibrium calculations and perfectly stirred reactor (PSR) simulations. By comparing the blend inhibitors with different blend ratios, it was found that the blend of C₃H₂F₃Br and C₂HF₅ at blend ratios of 25/75 and 50/50 effectively reduced the total heat release and system reactivity. In addition, the blend inhibitor not only weakened the fuel properties of C₂HF₅, but also further enhanced the bromine-catalysed radical recombination cycle. Notably, a new reaction occurred when C₃H₂F₃Br and C₂HF₅ were blended into the FAA-ACET chamber: Br + CHF₂CF₃ = HBr + CF₃-CF₂, indicating that the Br atoms promoted the decomposition of C₂HF₅.     

Activated air plasma flow generated by pulsed arc discharge for combustion enhancement

Important operating parameters, temperature characteristics and radical production in an air plasma flow generated by a low-power pulsed arc discharge were experimentally clarified for lean combustion enhancement and surface treatment. Furthermore, the time-dependent thermofluid field downstream from the torch was also clarified numerically and the downstream temperature well agreed with experimental data. Finally, the time evolution of production and decay of the chemical species in air plasma were clarified numerically under a high electric field.     

Experimental evaluation of methanol steam reforming reactor heated by catalyst combustion for kW-class SOFC

The distributed power generation of methanol steam reforming reactor combined with solid oxide fuel cell (SOFC) has the characteristics of outstanding economic advantages. In this paper, a methanol steam reforming reactor was designed which integrates catalyst combustion, vaporization and reforming. By catalyst combustion, it can achieve stable operation to supply fuel for kW-class SOFC in real time without additional heating equipment. The optimal operating condition of the reforming reactor is 523–553 K, and the steam to carbon ratio (S/C) is 1.2. To study the reforming performance, methanol steam reforming (MSR), methanol decomposition (MD), water-gas shift (WGS) were considered. Operating temperature is the greatest factor affecting reforming performance. The higher the reaction temperature, the lower the H₂ and CO₂, the higher the CO and the methanol conversion rate. The methanol conversion rate is up to 95.03%. The higher the liquid space velocity (LHSV), the lower the methanol conversion rate, the lowest is 90.7%. The temperature changes of the reforming reactor caused by the load change of stack takes about 30 min to reach new balance. Local hotspots within the reforming reactor lead to an excessive local temperature to test a small amount of CH₄ in the reforming gas. The methanation reaction cannot be ignored at the operating temperature. The reforming gas contains 70–75% H₂, 3–8% CO, 18–22% CO₂ and 0.0004–0.3% CH₄. Trace amounts of C₂H₆ and C₂H₄ are also found in some experiments. The reforming reactor can stably supply the fuel for up to 1125 W SOFC.     

Numerical analysis of methanol steam reforming reactor heated by catalytic combustion for hydrogen production

Thermal coupling of endothermic and exothermic reactions is an important pathway for integrated thermal management within a methanol steam reforming reactor heated by methanol catalytic combustion. In this study, a numerical model is developed for heat and mass transfer calculations, methanol steam reforming and catalytic combustion reactions, which is used to explore the effects of design parameters on compact parallel channel reactor performance. Efficiency of the integrated reactor is optimized by the coupling of endothermic and exothermic reactions using conventional wall material. Temperature uniformity is improved by the adjustment of the flow arrangement and the catalyst distribution. This work provides an effective energy management strategy and tool which can be adopted in the design of portable hydrogen generation systems.     

Detailed kinetic study of anisole pyrolysis and oxidation to understand tar formation during biomass combustion and gasification

Anisole was chosen as the simplest surrogate for primary tar from lignin pyrolysis to study the gas-phase chemistry of methoxyphenol conversion. Methoxyphenols are one of the main precursors of PAH and soot in biomass combustion and gasification. These reactions are of paramount importance for the atmospheric environment, to mitigate emissions from wood combustion, and for reducing tar formation during gasification. Anisole pyrolysis and stoichiometric oxidation were studied in a jet-stirred reactor (673–1173 K, residence time 2 s, 800 Torr (106.7 kPa), under dilute conditions) coupled with gas chromatography–flame ionization detector and mass spectrometry. Decomposition of anisole starts at 750 K and a conversion degree of 50% is obtained at about 850 K under both studied conditions. The main products of reaction vary with temperature and are phenol, methane, carbon monoxide, benzene, and hydrogen. A detailed kinetic model (303 species, 1922 reactions) based on a combustion model for light aromatic compounds has been extended to anisole. The model predicts the conversion of anisole and the formation of the main products well. The reaction flux analyses show that anisole decomposes mainly to phenoxy and methyl radicals in both pyrolysis and oxidation conditions. The decomposition of phenoxy radicals is the main source of cyclopentadienyl radicals, which are the main precursor of naphthalene and heavier PAH in these conditions.     

Conversion of hydrocarbons to synthesis gas in a counterflow moving bed filtration combustion reactor

The conversion of hydrocarbons to synthesis gases via partial oxidation in a non-premixed filtration combustion is considered in a continuous reactor wherein filtration medium/inert solid matrix comprises a moving bed of a granular material flowing countercurrently to the gas flow. Similar to the previously considered conversion in a reversed-flow reactor, such embodiment provides a possibility of attaining high combustion temperature due to efficient heat recuperation, while performing the process continuously without transients associated with the flow reverse. In the moving bed process, the flowrate of the granular material becomes an independent control parameter. A thermodynamic assessment of the macrokinetic conversion regimes as dependent on fuel composition and oxidant gas and fuel supply rates and granular solid flowrate is performed under the assumption that the combustion temperature is self-consistent according to thermodynamic equilibrium with the composition of syngas. Calculations for the methane/air-steam and 2-propanol/air-steam conversion are provided as examples. The calculations show that the process provides a possibility to combine in a stationary continuous process a high combustion temperature with low net heat effect and thus, a high chemical efficiency of conversion. The parametric domain for control parameters providing highly efficient conversion is determined.     

Modeling hydrogen production in a catalytic-inert packed bed reactor by rich combustion of heavy fuel oil

This work presents simulation results for the production of hydrogen by the rich combustion of heavy fuel oil in a dual zone packed bed reactor. The first zone provides catalytic-thermal cracking of the fuel and is followed by a second zone for partial oxidation reforming of the cracked products. The kinetic model for the heavy fuel oil reactions in the catalytic zone uses decalin as a model compound. The partial oxidation reforming zone uses model compounds for the product groups formed from decalin cracking, and uncracked decalin. The hybrid reactor model is compared to results from a model of an inert (non-catalytic) porous media reactor. The work considers equivalence ratios from 1 to 2, filtration velocities between 15.0 and 65.5 cm/s, heat loss from 10 to 108% and particle diameter between 3 and 7 mm, and evaluates their effect on conversion. The simulations with the hybrid reactor model, in slightly rich conditions (equivalence ratio = 1.3) and constant filtration velocity of 19.3 cm/s deliver maximum hydrogen production for an optimal length of the intermediate zone. Considering this optimization: the total energy conversion efficiencies improve with the increase of the equivalence ratio due to the presence of hydrocarbon species generated by the cracking process. It is observed that the hybrid reactor model makes a better use of vaporized fuel, compared to a model for an inert packed bed reactor, when the deposits of carbonaceous material in the latter exceed 7.4%.     

Enhancing efficiency of hydrocarbons to synthesis gas conversion in a counterflow moving bed filtration combustion reactor

An improvement is considered for the partial oxidation conversion of hydrocarbon gases to synthesis gas in a continuous non-premixed filtration combustion reactor with inert solid granular material flowing countercurrently to the gas flow. The reactor is supplemented with an additional heat exchanger, wherein the second reactant gas is preheated prior to supply to the middle part of the reactor. The composition of the gaseous products self-consistent with the temperature of combustion are assessed using approximation of established thermodynamic equilibrium in the products. The parametric domain for major control parameters, namely oxygen-to-fuel supply ratio, granular solid flowrate, and steam supply rate providing highly efficient conversion is determined. Calculations for the POX conversion of methane and a model biogas composition (50% methane, 40% carbon dioxide, 10% nitrogen) with air and steam are provided as examples. The calculations show that the process gives a possibility to substantially improve energy efficiency and provides a flexibility to control hydrogen yield through steam supply. The process provides a high chemical efficiency of conversion even with air used as an oxidant for conversion of low-caloric gases.     

New application of the fixed bed reactor to the measurement of coal reactivity under dynamic combustion conductions

A new laboratory scale facility (fixed bed reactor) for the determination of solid fuel reactivity is presented. The reactivity is measured by burning samples of partially pyrolysed coals under dynamic conditions (rapid change of temperature and oxygen concentration). Mass conversion rate is determined from heat and mass balance equations, and the influence of boundary layer diffusion and chemical reaction rate on the measured reaction rate is evaluated. In the scope of a co-operative test, the reactivity of Göttelborn bituminous coal has been investigated by different research institutions. Despite of the different conditions in different laboratory scale facilities, a good correspondence of results was found. Moreover, three coals of different rank have been investigated. The kinetic constants of these coals are also given.     

基于草木灰修饰铁矿石载氧体的串行流化床化学链燃烧实验

采用草木灰对铁矿石载氧体进行修饰，在1kWth串行流化床反应器上，以合成气（CO H2 CH4）为燃料进行了化学链实验，对其反应活性进行测试。结果表明：生物质灰修饰铁矿石载氧体在1 kWth串行流化床上表现出较高的反应活性，且反应温度越高，反应活性越好；燃料反应器出口CO和CH4体积百分数显著降低，930 ℃下分别为0.12%和2.63%，与采用纯铁矿石相比分别降低了97.3%和16.0%，CO2捕集效率提升明显，最大值达89.38%；SEM分析表明，反应后的铁矿石颗粒表面出现了晶粒熔融粘接的现象，但生物质灰修饰铁矿石的孔隙结构仍较为明显；EDS分析表明生物质灰修饰铁矿石中K的负载情况较为稳定，没有出现明显的流失现象。     

Catalytic decomposition of liquid hydrocarbons in an aerosol reactor: A potential solar route to hydrogen production

Catalytic decomposition of liquid fuels (n-octane, iso-octane, 1-octene, toluene and methylcyclohexane) is achieved in a continuous tubular aerosol reactor as a model for the solar initiated production of hydrogen, and easily separable CO free carbonaceous aerosol product. The effects of fuel molecular structure and catalyst concentration on the overall hydrogen yield were studied. Iron aerosol particles used as the catalysts, were produced on-the-fly by thermal decomposition of iron pentacarbonyl. The addition of iron catalyst significantly decreases the onset temperature of hydrogen generation as well as improves the reaction kinetics by lowering the reaction activation energy. The activation energy without and with iron addition was 260 and 100 kJ/mol, respectively representing a decrease of over 60%. We find that with the addition of iron, toluene exhibits the highest hydrogen yield enhancement at 900 °C, with a 6 times yield increase over thermal decomposition. The highest H₂ yield obtained was 81% of the theoretical possible, for n-octane at 1050 °C. The general trend in hydrogen yield enhancement is that the higher the non-catalytic thermal decomposition yield, the weaker the catalytic enhancement. The gaseous decomposition products were characterized using a mass spectrometer. An XRD analysis was conducted on the wall deposit to determine the product composition and samples for electron-microscopic analysis were collected exiting the furnace by electrostatically precipitating the aerosol onto a TEM grid.     

Conversion of jet fuel and butanol to syngas by filtration combustion

Replacing batteries with fuel cells is a promising approach for powering portable devices; however, hydrogen fuel generation and storage are challenges to the acceptance of this technology. A potential solution to this problem is on-site fuel reforming, in which a rich fuel/air mixture is converted to a hydrogen-rich syngas. In this paper, we present experimental results of the conversion of jet fuel (Jet-A) and butanol to syngas by non-catalytic filtration combustion in a porous media reactor operating over a wide range of equivalence ratios and inlet velocities. Since the focus of this study is the production of syngas, our primary results are the hydrogen yield, the carbon monoxide yield, and the energy conversion efficiency. In addition, the production of soot that occurred during testing is discussed for both fuels. Finally, an analysis of the potential for these fuels and others to be converted to syngas based on the present experiments and data available in the literature is presented. This study is intended to increase the understanding of filtration combustion for syngas production and to illuminate the potential of these fuels for conversion to syngas by non-catalytic methods.     

Investigation of combustion enhancement by ozone additive in CH4/air flames using direct laminar burning velocity measurements and kinetic simulations

The effect of ozone additive on the enhancement of the burning velocity for premixed methane–air flames is investigated by both experimental measurements and kinetic simulations. Laminar burning velocities with and without O₃ were directly measured using the Heat Flux method. The O₃ molecules were introduced into the system by a dielectric-barrier-discharge ozone generator installed in the O₂ gas line, which provided prompt control of on/off of the O₃ feed into the system, enabling a precise comparison of the measured burning velocity with and without ozone additives. Noticeable burning velocity enhancement was observed at off-stoichiometric conditions rather than stoichiometric conditions. With 3730 ppm O₃ additive in the oxidizer, experimental data shows ～8% burning velocity increase in fuel-rich mixtures and ～3.5% burning velocity increase for the stoichiometric mixture. With 7000 ppm ozone additive in the oxidizer, maximum ～16% burning velocity increase was observed at fuel-lean conditions while ～9.0% was found at fuel-rich conditions. An O₃ kinetic mechanism involving 16 elementary reactions together with the GRI-Mech 3.0 was composed and validated through CHEMKIN calculations, which gives good predictions of the burning velocities with and without O₃ additives. Extra O radicals contributed by O₃ molecules in the pre-heat zone initiate and accelerate the chain-branching reactions and consequently increase the burning velocity.     

On the contribution of external cost calculations to energy system governance: The case of a potential large-scale nuclear accident

The contribution of nuclear power to a sustainable energy future is a contested issue. This paper presents a critical review of an attempt to objectify this debate through the calculation of the external costs of a potential large-scale nuclear accident in the ExternE project. A careful dissection of the ExternE approach resulted in a list of 30 calculation steps and assumptions, from which the 6 most contentious ones were selected through a stakeholder internet survey. The policy robustness and relevance of these key assumptions were then assessed in a workshop using the concept of a ‘pedigree of knowledge’. Overall, the workshop outcomes revealed the stakeholder and expert panel's scepticism about the assumptions made: generally these were considered not very plausible, subjected to disagreement, and to a large extent inspired by contextual factors. Such criticism indicates a limited validity and useability of the calculated nuclear accident externality as a trustworthy sustainability indicator. Furthermore, it is our contention that the ExternE project could benefit greatly – in terms of gaining public trust – from employing highly visible procedures of extended peer review such as the pedigree assessment applied to our specific case of the external costs of a potential large-scale nuclear accident.     

Using steam reforming to produce hydrogen with carbon dioxide capture by chemical-looping combustion

In this paper, a novel process for hydrogen production by steam reforming of natural gas with inherent capture of carbon dioxide by chemical-looping combustion is proposed. The process resembles a conventional circulating fluidized bed combustor with reforming taking place in reactor tubes located inside a bubbling fluidized bed. Energy for the endothermic reforming reactions is provided by indirect combustion that takes place in two separate reactors: one for air and one for fuel. Oxygen is transferred between the reactors by a metal oxide. There is no mixing of fuel and air so carbon dioxide for sequestration is easily obtained. Process layout and expected performance are evaluated and a preliminary reactor design is proposed. It is found that the process should be feasible. It is also found that it has potential to achieve better selectivity towards hydrogen than conventional steam reforming plants due to low reactor temperatures and favorable heat-transfer conditions.     

Thermal analysis of a micro tubular reactor for methanol steam reforming by optimizing the multilayer arrangement of catalyst bed for the catalytic combustion of methanolr

Packed bed tube reactors are commonly used for hydrogen production in proton exchange membrane fuel cells. However, the hydrogen production capacity of methanol steam reforming (MSR) is greatly limited by the poor heat transfer of packed catalyst bed. The hydrogen production capacity of catalyst bed can be effectively improved by optimizing the temperature distribution of reactor. In this study, four types of reactors including concentric circle methanol steam reforming reactor (MSRC), continuous catalytic combustion methanol steam reforming reactor (MSRR), hierarchical catalytic combustion methanol steam reforming reactor (MSRP) and segmented catalytic combustion reactor with fins (MSRF) are designed, modeled, compared and validated by experimental data. It was found that the maximum temperature difference of MSRC, MSRR, MSRP and MSRF reached 72.4 K, 58.6 K, 19.8 K and 11.3 K, respectively. In addition, the surface temperature inhomogeneity U_f and CO concentration of the MSRF decreased by 69.8% and 30.7%, compared with MSRC. At the same reactor volume, MSRF can achieve higher methanol conversion rate, and its effective energy absorption rate is 4.6%, 3.9% and 2.6% higher than that of MSRC, MSRR and MSRP, respectively. The MSRF could effectively avoid the influence of uneven temperature distribution on MSR compared with the other designs. In order to further improve the performance of MSRF, the influences of methanol vapor molar ratio, inlet temperature, flow rate, catalyst particle size and catalyst bed porosity on MSR were also discussed in the optimal reactor structure (MSRF).     

Measurement of heat transfer enhancement in forced convection due to hydrogen bubbles produced by electrolysis

Measurements of heat transfer were performed for a circular cylinder in cross-flow with hydrogen bubbles uniformly generated by electrolysis. The heat transfer coefficient as a function of velocity and temperature differences between the wall and the fluid is presented.Measurements in the low-velocity range show a heat transfer increase due to the stirring action of the bubbles.The use of this technique to investigate bubble dynamics and boiling heat transfer is discussed.     

Control of methane flame properties by hydrogen fuel addition: Application to power plant combustion chamber

This study presents a numerical investigation of the effects of mixing methane/hydrogen on turbulent combustion processes taking place in a burner similar to that integrated in gas turbine power plants. Thereby, in comparison to the reference case where the burner is fuelled by 100% of methane, the variations of the axial velocity field, temperature field and mass fraction of carbon monoxide field are examined for different percentages of hydrogen fuel injection. The computed results, obtained by using the software Fluent-CFD, are compared and validated against experimental reference data. Results show that the hydrogen addition to the methane has an impact on all physical and chemical parameters of the reactive system.     

Formulation reproducing the ignition delays simulated by a detailed mechanism: Application to n-heptane combustion

This article is part of the project to model the kinetics of high-temperature combustions, occurring behind shock waves and in detonation waves. The “conventional” semi-empirical correlations of ignition delays have been reformulated, by keeping the Arrhenius equation form. It is shown how a polynomial with 3^N coefficients (where N∈[1,4] is the number of adjustable kinetic parameters, likely to be simultaneously chosen among the temperature T, the pressure P, the inert fraction X_Ar, and the equivalence ratio Φ) can reproduce the delays predicted by the Curran et al. [H.J. Curran, P. Gaffuri, W.J. Pitz, C.K. Westbrook, Combust. Flame 129 (2002) 253-280] detailed mechanism (565 species and 2538 reactions), over a wide range of conditions (comparable with the validity domain). The deviations between the simulated times and their fits (typically 1%) are definitely lower than the uncertainties related to the mechanism (at least 25%). In addition, using this new formalism to evaluate these durations is about 10⁶ times faster than simulating them with Senkin (Chemkin III package) and only 10 times slower than using the classical correlations. The adaptation of the traditional method for predicting delays is interesting for modeling, because those performances are difficult to obtain simultaneously with other reduction methods (either purely mathematical, chemical, or even mixed). After a physical and mathematical justification of the proposed formalism, some of its potentialities for n-heptane combustion are presented. In particular, the trends of simulated delays and activation energies are shown for , , XAr∈[0,0.7], and Φ∈[0.25,4.0].     

Reduction of the NOx emission of a closed combustion chamber by changing to air flows with swirl

From a study of NO_x production in flames it follows that NO_x emission differs for various combustion chambers. In addition to increasing the combustion chamber temperature or the volume flow into the combustion chamber, it is essential to find another way of reducing the production of NO_x in the combustion process. In this paper the effect of swirl on NO_x production is considered using a combustion chamber having air entry first without and then with swirl. Graphs show the influence of swirl on NO_x reduction.