Heterogeneous Combustion Research Articles

Hess Law Applicability to Heterogeneous Combustion Reactions Resulting to Non-Monotonicity of Enthalpy Distribution in Them

The definition of reaction irreversibility (reversibility) available now in literature does not allow for determining this reaction peculiarity before a practice. Moreover, in mathematical terms, such a definition is not universal and leads actually to the denial of any similarity in physicochemical processes, in particular in combustion waves. We have considered the irreversibility of reactions from the mathematical viewpoint as independence of a reaction rate on the concentration of products directly. As a result, in terms of Lei numbers, we have formulated conditions sufficient for the enthalpy of reagents to be a complete differential (or a thermodynamic potential) and independent of the reaction path. And conversely, the conditions necessary for that enthalpy of reagents are significantly dependent on the reaction path (Le ≌ 1). A comparison of the conclusions we obtained with the data from experiments available in the literature showed a good agreement with them and put an edge on the question of determining the irreversibility (reversibility) of reactions in the scientific community. We have shown that the small values of the diffusion coefficients of reagents Di compared to the thermal diffusivity coefficient κ, and with them, the corresponding small Lewis numbers (Lei = Di/κ << 1) for the most heterogeneous combustion synthesis systems, are the sufficient conditions for the enthalpy of reactions (H) to be a complete differential of the systems states and, vice versa, a strong similarity appears between the enthalpy and concentration fields at Le ≌ 1.

Experimental study on improving the efficiency of hydrogen production by partial oxidation of ethanol

Bioethanol as the renewable biomass fuel gradually becomes a promising feedstock for hydrogen production. To improve the efficiency of hydrogen production, a typical double-layer porous media burner was established for the partial oxidation reaction of ethanol (POE). The effects of porous media structure, initial ethanol and air conditions on the temperature distribution and gas production were investigated based on the start-up characteristics of the burner. The results indicate that the lowest start-up time (2400 s) and best hydrogen production (9.80%) were obtained by filling the downstream section with 8 mm Al2O3 pellets and introducing ethanol at the upstream inlet, compared with that of 6 and 10 mm pellets. The productions of methane and hydrogen were improved to some extent by adding the water with 10% fraction. And the highest concentration of hydrogen was achieved at the air velocity of 8 cm/s. When the O2/C ratios were 0.2 and 0.25, the maximum hydrogen yield (23.28%) and ethanol conversion (77.42%) were obtained respectively.

Formation and Combustion Heat Release of Naphthenic-Based Crude Oil Cokes at Different Reaction Temperatures.

Petroleum cokes prepared from naphthenic crude oil differ significantly in terms of the oxygen content and hydrogen/carbon (H/C) ratio, which mainly depend on the different coking temperatures. Thermogravimetric-differential scanning calorimetry was applied to study the heat release and combustion weight loss of petroleum cokes prepared at 350 and 500 °C, respectively. The effect of different coke formation temperatures on the combustion properties of the coke formed during air injection in situ combustion (ISC) was also investigated. The results showed that the petroleum coke formed under oxygen exhibited an H/C ratio of 0.895 and an O/C ratio of 0.109 at 350 °C and an H/C ratio of 0.395 and an O/C ratio of 0.054 at 500 °C. As the temperature rises, the hydrogen atoms on the petroleum coke molecules intensify to separate and form water molecules and thus giving off heat. It can be further inferred that under the combustion temperature of air injection ISC, the coke at 350 °C can release more heat in the lower combustion temperature range, and the combustion weight loss is faster; however, the formation temperature continues to rise due to combustion at 500 °C, coke begins to release massive heat, and the combustion weight loss is as high as 97.95%. The combustion residuals of both temperature cokes and the residual solid content of the formation after combustion in porous media are both little, which can be used as fire flooding fuels at different formation temperatures to provide heat energy for oil displacement.

Combustion characteristics of aluminum particle jet flames in a hot co-flow

Aluminum(Al) powder has been proposed as a promising dense energy carrier to store and transport abundant renewable power for a future carbon-neutral society. In this work, a detailed model for simulating Al particle cloud combustion with multiple oxidizers in the context of large eddy simulation is developed. The Al particles are tracked in the lagrangian framework with various sub-models including phase change, heterogeneous surface reaction, evaporation and radiation. The model is then employed to simulate turbulent jet flames with Al particles(d32=25μm) in a hot co-flow. Tunable diode laser absorption spectroscopy(TDLAS) is used to measure the temperature and H2O distribution. The present model is first validated by comparing the predicted condensed Al2O3 distribution with the measured Mie scattering signals. The results indicate that the simulation results are in good agreement with the experimental measurements. Further comparisons of the temperature and H2O profile are then conducted. The numerical simulation predicts weaker mass and heat transport in the upstream and middle of the flame along the centerline of the burner, compared with the reconstructed TDLAS results, but the downstream scalar profiles of the jet flame agree well with the measurements. Discrete characteristics are observed in the simulated instantaneous temperature and condensed Al2O3 snapshots, which are due to the nature of heterogeneous combustion of metal fuel particles under fuel-lean conditions. Finally, the influence of the oxidizer mole fraction and the initial temperature of the primary jet on the ignition distance is analyzed. The results demonstrate that the ignition distance can be decreased effectively by increasing the oxidizer concentration of the hot co-flow or the initial temperature of the Al particles and carrier gas.

Turbulent flame propagation mechanism of polymethylmethacrylate particle cloud–ammonia co-combustion

Ammonia is a promising energy carrier for realizing a carbon-neutral society. In particular, solid particle cloud–ammonia co-combustion is considered as an efficient and feasible method to reduce CO2 emissions from the thermal power generation sector by using particle-fuel such as solid waste, biomass, and pulverized coal particles in combustors. However, the fundamental turbulent flame propagation mechanism of solid particle cloud–ammonia co-combustion remains unknow. Therefore, the present study intends to investigate and validate the general turbulent flame propagation mechanism of solid particle cloud–ammonia co-combustion. To achieve this aim, silica particle cloud–ammonia–oxygen–nitrogen mixing combustion, silica particle cloud–acetylene–air mixing combustion, and PMMA particle cloud–ammonia–oxygen–nitrogen co-combustion experiments were conducted. The results showed that the turbulent flame propagation velocity of silica particle cloud–gas-fuel–oxidizer mixing combustion is lower than that of pure gas-fuel–oxidizer combustion. However, the comparison of the turbulent flame propagation velocity of PMMA particle cloud–ammonia co-combustion and that of pure ammonia combustion, showed that whether the flame propagation of the co-combustion was higher than that of the pure ammonia combustion was dependent on the equivalence ratio of the ammonia-oxidizer. Therefore, the consistency of the results between the current study of PMMA particle cloud–ammonia co-combustion and the previous study for coal particle cloud–ammonia co-combustion indicates the turbulent flame propagation mechanism of solid particle cloud–ammonia co-combustion is dominated by the negative effect of the heat sink by unburned particles and the local equivalence ratio increment effect in the preheat zone of the flame front by the addition of the volatile matter, and that the positive effect of radiation from soot particles has little effect on the turbulent flame propagation of co-combustion for small-scale flames. Further, the influence of the heterogeneous combustion of char particles on the turbulent flame propagation of solid particle cloud–ammonia co-combustion is minor because of its slow combustion process. Based on the validated turbulent flame propagation mechanism of co-combustion, new numerical simulation models for solid particle cloud–ammonia co-combustion can be developed in the future.

Enhancement of partial oxidation reformer by the free-section addition for hydrogen production

Hydrogen production by the partial oxidation combustion in porous media provides an efficient utilization for renewable biogas. In this paper, the free section with lengths of 0, 5, 10, 15 mm were designed in the middle of two-section porous media respectively. The effects of free section addition on the temperature distribution and methane conversion were investigated to determine the optimal burner parameters at the operating conditions of different velocities and equivalence ratios. Results indicate that the highest temperature appeared in the free length of 10 mm but the maximum of H2 energy conversion efficiency was in that of 15 mm. For the Al2O3 ceramic foam of 20 PPI, the better heat recirculation and preheating effect were shown between the gas and solid. However, the productions of H2 and CO at the 10 PPI burner reached the highest. With the increasing of equivalence ratio from 1.4 to 1.8, the energy conversion efficiency of syngas rose from 41% to 61% with the 15 mm free length. The appropriate free section addition contributes to improving the reforming efficiency, which provides a new way for the burner design.

Effects of Key Influencing Factors on the Flame Inclination of Low Concentration Methane (LCM) Combustion in Porous Burner

ABSTRACT In present work, a local thermal non-equilibrium numerical model of low concentration methane combustion in porous media is established. The modified thermal effective thermal conductivity of foam ceramics was introduced. The heat and mass dispersion effect as well as the influence of porous media structure on the heat transfer were fully considered. Studies found that the combustion flame front of low concentration methane in porous media always show a S-shaped state under special conditions. The influencing factors of inlet velocity, methane concentration, wall heat loss, and the porous burner size were analyzed. Results show that without initial thermal perturbation and the absence of defects in the porous media, the flame front in porous burner still has the possibility of inclination. As the inclination of the flame front continues to increase, the flame keeps approaching the burner outlet until blow off. Among the considered four influencing factors, the size of porous burner has the greatest influence on the inclination of flame front. As the burner diameter increases from 0.04 m to 0.12 m, the inclination angle of the flame front increases continuously, and the maximum inclination angle is 36°. The flame inclination will cause uneven temperature distribution downstream of the burner and local heat concentration. With the increase of wall heat loss, the high-temperature zone downstream of the burner will evolve into a local high-temperature zone near the flame front. With the reducing of porous burner diameter, the inclinational instability could be suppressed to some extent.

Heterogeneous propellant combustion with conjugated interface conditions

A numerical framework for three-dimensional simulation of heterogeneous propellant combustion with conjugated interface conditions is proposed. Evolutions of the solid and gas phase are resolved in their respective but overlapping grids. The two-way coupling between the solid and gas phase is realized by performing the interface conditions on the regression surface using the level-set method. The morphology of the propellant is characterized by ingredients generated using a random packing algorithm. The conservation of mass as well as energy in the gasification process can be ensured by imposing appropriate source terms in the immersed boundary cells in the gas phase. The obtained test case results match the experimental trend satisfactorily, which justifies the validity of the present algorithm designed for solid fuel combustion problems.

Experimental study on control of flame inclination in a thermal flow reversal reactor with extra lean premixed methane/air intake

Combustion instability is a basic obstacle to the wide application of porous media combustion devices in industry. However, the investigation about it with extremely dilute gas fuel issorelylacking due to the limitation of laboratory test conditions. In this study, inclinational instability of extra lean-premixed flame (with volume fraction below 1%) in a thermal flow-reversal reactor with honeycomb ceramic packings is investigated experimentally. The effects of inlet methane concentration on the flame inclination are studied. Then the flame inclination is restrained through electronically controlled adjustment on the valve opening and switch time. The experimental results show that the intake methane concentration has a remarkable effect on the flame inclination angle variation in each semi cycle. When the intake concentration decreases from 0.6 vol% to 0.2 vol% and the flow direction is downward, the growth of the flame inclination angle in each semi cycle gets lower. The adjustment on the scale of the high temperature zones in two half parts of the packed bed through the valve opening has a great influence on the flame front position and flame inclination angle. The setting of asymmetric switch time for the reverse flow also has obvious effect on the evolution of the flame inclination. Finally, the flame inclination angle drops firmly for approach of the two high temperature zones in size and height. The synchronous adjustment of valve opening and switch time fastens the convergence of the two high temperature zones in two half parts of the packed bed. This work is beneficial to the development of thermal flow-reversal reactor technique for better dealing with ventilation air methane and other lean gaseous fuel streams.

Combustion regimes in inert porous media: From decoupled to hyperdiffusive flames

Based on the classical volume-averaged equations, this work proposes a classification of porous media combustion in three distinct regimes for increasing interphase heat transfer, namely: decoupled, intermediate and hyperdiffusive. In the decoupled regime, flames behave as preheated free-flames. In the intermediate regime, large superadiabaticities are found. In the hyperdiffusive regime, flames are governed solely by an increase in thermal conductivity. The transition between these regimes is shown to be driven by two dimensionless parameters. The extent of the intermediate regime and the maximal superadiabaticity are proven to be related to the ratio between the diffusive and reactive length scales of the reference free-flame. Eventually, it is emphasized how the heat-recirculating system acts locally as a Lewis-changing device, whose effect harmonizes in the hyperdiffusive regime.

Boundary layer combustion of paraffin fuels for hybrid propulsion applications

Droplet entrainment offers paraffin fuels a regression rate of 3–4 times higher than classical polymer fuels, which is favorable for the hybrid rocket motor, however, the heterogeneous combustion reactions between droplets and oxidizer lead to a more complicated combustion mechanism. Combustion diagnosis was carried out for paraffin fuels combusted in a windowed 2D slab burner, and the boundary layer combustion was investigated by using high speed camera and schlieren in this study. The results showed that the presence of remarkable periodic flame and chamber pressure oscillation was due to the periodic accumulation and breakdown of liquid film layer; Kelvin−Helmholtz instability was a necessary but not the sufficient condition for droplet entrainment, and the critical oxidizer mass flux of Kelvin−Helmholtz instability and droplet entrainment was about 12.31 kg m−2 s−1 and 30.15 kg m−2 s−1; the thickness of turbulent boundary layer and the height of combustion flame were inversely correlated with the oxidizer mass flux, indicating the more efficient combustion under higher oxidizer mass flux; the entrainment droplets disappeared at elevated chamber pressures due to the inhibition of Kelvin−Helmholtz instability, meanwhile, the thickness of turbulent boundary layer was found to increase; entrainment droplets increased and the height of combustion flame decreased due to the acceleration of combustion reactions when the copper chromite was used as the combustion catalyst, which favored the enhancement of the regression rate of fuels and the combustion efficiency of hybrid rocket motors.

Comprehensive investigations on the explosion suppression of biomass fuels: Starch as a representative

Starch is one type of biomass fuel raw material commonly used, but presents deflagration hazards when suspended in the air. Dust deflagration accidents might be effectively inhibited by creating a gas inerting atmosphere. In this work, the evolution of starch deflagration flame in the inhibitory gas atmosphere N2 or CO2 was investigated. Under the action of inerting gas, heterogeneous combustion is strengthened, the flame propagation velocity is reduced and the acceleration trend is smooth. The maximum flame temperature is decreased and fluctuation after reaching the maximum value is weakened. According to the micro characterization of deflagration residues, after injecting inhibitory gas, more amorphous carbon is protected and the carbon layer formed has a smaller crystal size, some easily oxidized functional groups in starch are maintained during the deflagration. Meanwhile, carbon content and the original chemical bond ratio in the residues are also increased. In addition, experimentally measured the maximum limit inerting ratios of CO2 and N2 are chosen as 33% and 54%, respectively. On this basis, safety application for gas inerting technology in practical processes was discussed.

<i>In situ</i> study of heterogeneous media combustion processes by time Resolved XRD

The use of in situ diagnostic methods is required to obtain data on the structure and composition of the material in real time when studying condensed substances in the course of chemical reactions and phase transformations. We present the results on the development and application of the time-resolved X-ray diffraction method for the diagnosis of fast processes in heterogeneous condensed media, including self-propagating high-temperature synthesis. An X-ray tube was used as a radiation source. The method is based on the use of high-speed registration of XRD patterns in a wide angular interval and provides obtaining in situ data on the evolution of the crystal structure of the reactants. A high-speed linear detector and reaction chambers, an effective analysis system with a time resolution in the range of 10–1 – 102 sec has been developed on the base of a powder diffractometer. A complex of object-oriented diffraction techniques has been developed to study the dynamics of phase transitions in self-propagating high-temperature synthesis of inorganic materials, combustion of energy systems and liquid solutions, crystallization of amorphous alloys, in the analysis of the phase composition of materials in the process of heat treatment. The results obtained can be used to identify the mechanism of structural and chemical transformations in condensed matter.

Hollow Spheres as Inert Packed Bed from Lean to Rich Combustion in Porous Media

Hollow Spheres as Inert Packed Bed from Lean to Rich Combustion in Porous Media

Design of porous media burner control system based on arm embedded processor

Because the traditional porous media burner control system has the problems of low control accuracy and long control time, a porous media burner control system based on Advanced RISC Machines (ARM) embedded processor is designed. The hardware of the system is designed through ARM embedded processor, FPGA data acquisition card, power supply circuit, reset circuit and audible and visual alarm module, and the system software is designed based on it. According to the structure of fuzzy controller, fuzzy control rules, assignment table of fuzzy variables, fuzzy control table, and defuzzification, the fuzzy controller is used to fuzzy control the porous media burner. The simulation results show that the designed system has the highest precision and the shortest control time.

Effects of Infrared Radiant Coating on Porous Media Combustion

Effects of Infrared Radiant Coating on Porous Media Combustion

Effects of Infrared Radiant Coating on Porous Media Combustion Using Simulated Natural Gas and Air Mixture

Effects of Infrared Radiant Coating on Porous Media Combustion Using Simulated Natural Gas and Air Mixture

Numerical Investigation of Premixed Ammonia-Hydrogen Combustion within Porous Inert Media Burner

Numerical Investigation of Premixed Ammonia-Hydrogen Combustion within Porous Inert Media Burner

Numerical simulation of aluminum dust counterflow flames

<sec>Aluminum is widely used as an additive in solid rocket propellants and pyrotechnics due to its outstanding characteristics such as high energy density and combustion temperature, environmentally benign products, and good stability. Recently, aluminum powders are found to present great potential serving as alternative fuel in a low-carbon economy. In this paper, a detailed model including the effects of interphase heat transfer, phase change, heterogeneous surface reactions, homogeneous combustion and radiation is employed to investigate aluminum dust counterflow flames.</sec><sec>The numerical model is first validated by simulating the aluminum dust counterflow flames of McGill University. The results indicate that the particle velocity profile is in very good agreement with the experimental measurements. A detailed analysis of estimating the gas phase velocity based on the particle velocity is performed by using Stoke time <i>τ</i><sub>s</sub>. The results show that a correct value of <i>τ</i><sub>s</sub> is the key to this method, and using a single value of <i>τ</i><sub>s</sub> can bring a notable bias to the results, which may also affect the evaluation of flame speed from the counterflow flame. It is suggested that model validation should be carried out by directly comparing the particle velocity profiles from the simulations with those from the experiments. The flame structure of the aluminum dust counterflow flame is discussed, and the interphase heat transfer model is found to have a great influence on the flame for particle sizes smaller than 10 μm. Therefore, when simulating the aluminum dust flames with small particle sizes, the interphase heat transfer model should be selected carefully so that it can cover the transition heat transfer regime. The effect of particle diameter is examined. With the increase of the particle size, the flame speed continues to decrease, and most particles with a diameter of 15 μm cannot be fully burnt in the present configuration. The strain rate is found to be an important factor affecting the dust flame. As the strain rate increases, the residence time of the particles in the flame zone decreases, which ultimately leads the particles to be combusted incompletely. Moreover, the reaction zone of the counterflow flame, marked as AlO, is observed to be shrunk from a large double-peak structure into a small single-peak one along the burner centerline when strain increases. The reference flame speed increases with strain rate, and an unstretched reference flame speed of roughly 29 cm/s can be obtained by linear extrapolation of the predicted results. The effect of radiation is investigated by comparing two cases with and without radiative heat transfer. The results show that the heat loss caused by radiation can lead the temperature to decrease greatly in the gas phase, but the heating effect on the particles by radiation is relativelysmall.</sec>

NUMERICAL STUDY OF A RECIPROCAL FLOW POROUS MEDIA BURNER USING A TURBULENCE MODEL

NUMERICAL STUDY OF A RECIPROCAL FLOW POROUS MEDIA BURNER USING A TURBULENCE MODEL