Heterogeneous Combustion Research Articles

Effects of ratio of premixed mixtures on combustion characteristics in heterogeneous combustion field by using multi-stage impinging injection system

Effects of ratio of premixed mixtures on combustion characteristics in heterogeneous combustion field by using multi-stage impinging injection system

Heterogeneous reaction and homogeneous flame coupled combustion behavior of n-decane in a partially packed catalytic bed combustor

The premixed combustion experiments of n-decane were conducted in micro-combustors. The characteristics and mechanism of three combustion modes were studied: homogeneous combustion (HMC), heterogeneous combustion (HTC), and coupled combustion (CC) with both HTC and HMC. The homogeneous flame in CC cases can be established downstream of catalytic bed. The stable combustion range of CC is near the stoichiometric ratio, and the lean/rich limits of equivalence ratio (Φ) are close to the lean limit of the HMC and the rich limit of the HTC respectively. The HTC cases show high CO2 selectivity, and CO2 is the major products even at Φ > 1. The different mechanisms of O2 and n-decane adsorption on the Pt active site have significant effects on the stable limits of HTC. Hot flame in the fire core is followed by warm flame in the HMC cases. When Φ > 0.9, n-decane can’t burn out in so small hot flame that the effect of warm flame becomes obvious, resulting in some combustible species being generated in products. CO becomes the main product instead of CO2 in the HMC cases, when Φ > 1.6. In the CC cases, high n-decane conversion owes to the coupling of homogeneous flame and heterogeneous reaction. The homogeneous flame presents warm flame properties at Φ = 1.3 ~ 1.4, and hot flame properties at Φ = 1. More importantly, compared with HMC and HTC, much longer combustion zone achieved in CC with more even distribution of wall temperature and without obvious hot spots, which is significative to actual applications like the thermo-photovoltaic system.

Characteristics of large-area porous media burner applicable to direct-fired non-oxidizing annealing furnace

In the continuous steel strip annealing process, uniform and rapid heating in a non-oxidizing atmosphere is required to improve product quality and productivity while reducing emissions and saving energy. In this study, the combustion characteristics of a porous media burner (PMB) with a large surface area (40 × 25 cm2) are studied for application in a non-oxidizing annealing furnace using a model furnace. The furnace temperature is controlled according to the variation of the thermal load per unit area (475–928 kW/m2) and equivalence ratio (1.05–1.35), and its range was 1130–1330 °C at a 0% oxygen concentration or non-oxidizing atmosphere. In high furnace temperature conditions, the PMB surface temperature and radiant heat flux are also measured. Based on the results, the combustion mode changes, which implies a transition from the surface to the submerged combustion mode of the PMB, and were directly compared with the crossover temperature. The criteria for the transition of the PMB combustion mode are also proposed using basic radiation theory. Moreover, the effective emissivity of the porous media is evaluated from the measurement of the radiant heat flux, and its value was 0.845. Finally, the PMB durability is evaluated through a cyclic on-off operation test and consequently, the operational safety was confirmed.

Classification and evolution of bifurcation curves for a porous-medium combustion problem with large activation energy

We study the classification and evolution of bifurcation curves for the porous-medium combustion problem \begin{document}$ \begin{equation*} \begin{cases} u^{\prime \prime }(x)+\lambda \dfrac{1+au}{1+e^{d(1-u)}} = 0, \ -1<x<1, \\ u(-1) = u(1) = 0, \end{cases} \end{equation*} $\end{document} where $ u $ is the solid temperature, parameters $ \lambda >0 $, $ a\geq 0 $, and the activation energy parameter $ d>0 $ is large. We mainly prove that, on the $ (\lambda , ||u||_{\infty }) $-plane, the bifurcation curve is S-shaped with exactly two turning points for any$ \ (d, a)\in \Omega \equiv \left \{ (d, a):(0<d<d_{1}, \text{ }a\geq A_{1}(d))\text{ or }(d\geq d_{1}, \text{ }a\geq 0)\right \} $ for some positive number $ d_{1}\approx 2.225 $ and a nonnegative, strictly decreasing function $ A_{1}(d) $ defined on $ (0, d_{1}]. $ Furthermore, for any$ \ (d, a)\in \Omega , $ we give a classification and evolution of totally four different S-shaped bifurcation curves. In addition, for any $ d>0 $ and $ a\geq \tilde{a}\approx 1.704 $ for some positive $ \tilde{a}, $ then the bifurcation curve $ S $ is type 4 S-shaped on the $ (\lambda , \left \Vert u\right \Vert _{\infty }) $-plane.

Experimental research on porous media combustion of SOFC exhaust gas

The exhaust gas temperature of SOFC is as high as 600–1000 °C and contains combustible components that can be reused. In this paper, an experimental system was set up to study the combustion characteristics of SOFC simulated exhaust gas in a porous medium burner, the combustion of SOFC exhaust gas under different equivalence ratio, the effects of hydrogen concentration and water vapor fraction in the anode exhaust gas and the stable combustion limits of the burners had been analyzed. Results showed that: equivalence ratio and adiabatic flame temperature had a significant effect on the combustion, when the adiabatic flame temperature of mixed gas below 440 °C, or the water vapor content in the anode gas was greater than 85%, the flame couldn’t be stably burned; On the condition of the fuel utilization rate of 60% and the equivalence ratio of 0.40, the burner could burn stably with the inlet airflow velocity from 0.16 to 0.24m/s; The combustion stability limit of the exhaust gas burner varied with the utilization ratio of the stack, with increasing of fuel utilization ratio, the equivalence ratio of lifting and flashback limit was changing, and the adiabatic flame temperature of stable combustion limit had been studied.

Thermal oxidation and heterogeneous combustion of AlH3 and Al: A comparative study

Aluminum hydride (AlH3) is currently receiving considerable attention due to its potential to replace aluminum (Al) as a new metal fuel in solid propellants. In this study, simultaneous thermogravimetric analysis and differential scanning calorimetry and a laser ignition testing system were used to investigate the thermal oxidation and combustion characteristics of AlH3 and Al. The combustion residues were then analyzed using a scanning electron microscope. Results show that the thermal oxidation processes of both AlH3 and Al contain a mass loss stage and two mass gain stages. After dehydrogenation, AlH3 exhibits a stepwise oxidation behavior that is similar to that of a similar sized Al, but has a much higher mass gain. The combustion process of Al can be divided into three stages, which correspond to the combustion of dispersed Al particles, formation of Al agglomerate and combustion of Al agglomerate, respectively. The combustion flame of AlH3 is significantly different from that of Al, but their microscopic combustion behaviors are very similar. A unique flame delamination phenomenon exists in the early stage of AlH3 flame development. AlH3 has shorter ignition delay times and a larger combustion intensity than Al. The release and combustion of hydrogen changes the combustion environment of the remaining AlH3 particles, leading to the appearance of an OH characteristic peak and increasing the production path of AlO. The morphology of combustion residues shows that the dehydrogenation of AlH3 starts from the particle surface and then gradually expands towards its interior.

Control Strategies on HCCI Engine Performance and Emission characteristics by Combined Effect of Exhaust Gas Recirculation with Blend of Biodiesel and N-Heptane

ABSTRACT The usage of Tamanu methyl ester (TME) in a conventional CI engine produces more NO x emission because of in-built oxygen content in biodiesel. Simultaneous reduction of NO x and smoke is a really difficult way in a conventional CI engine. The usage of biodiesel in the HCCI engine would be a better way to reduce smoke and NO x emission due to the homogenous mixture rather than heterogeneous combustion and lower in-cylinder temperature. In this study, the experimental analysis was carried out on a single-cylinder engine operated in HCCI mode. The test fuels used for this experiment are TME 90% + n-heptane 10% (TME90nH10), TME 80% + n-heptane 20% (TME80nH20), TME 70% + n-heptane 30% (TME70nH30), and TME 60% + n-heptane 40% (TME60nH40). The test results noticed that brake thermal efficiency (BTE) of TME70nH30 showed higher when compared to neat TME, diesel, and other blends. The CO, HC, and smoke emission were decreased by 3.1%, 6.5%, and 9.2%, respectively, for a blend TME60nH40 when compared to TME70nH30. The smoke and UBHC emission of TME60nH40 was 23.5% and 26.4% lower than the HCCI mode of operation in diesel fuel. When n-heptane concentration increases beyond 40% in the blend, the performance characteristics of the engine started to decrease due to the high cetane rate of n-heptane which leads to shortening the span of the autoignition period that results in too advanced combustion. Based on emission characteristics of the engine, TME60nH40 was chosen to be the best blend and further study was carried out using TME60nH40 at different exhaust gas recirculation (EGR) percentages. The engine was operated with TME60nH40 with different EGR percentages of 5, 10, and 15. With the increase in the percentage of EGR increases the CO and UBHC emissions but on the other side, NO x emission was drastically decreased.

Pore-resolved simulations of porous media combustion with conjugate heat transfer

Porous media combustion (PMC) is an active field of research with a number of potential advantages over free-flame combustors. A key contributor to these phenomena is the interphase heat exchange and heat recirculation from the products upstream to the reactants. In this paper, we present a network model that captures the conjugate heat transfer in pore-resolved 2D simulations of PMC. A series of simulations are presented with varying solid conduction and inlet velocity to isolate the role of conjugate heat transfer on the salient features of the burner, including flame stability, axial temperature profiles, and flame structure. We show that both the flame stabilization and the propagation behavior are strongly related to the conjugate heat transfer, and the flame stability regime is shifted to higher velocities as the conductivity of the solid material is increased.

Study of pore-scale coke combustion in porous media using lattice Boltzmann method

In-situ combustion (ISC) for heavy oil recovery is a high-risk process and thus calls for a deep understanding of the coke combustion front. However, simulations for describing pore-scale coke combustion are challenging because of complicated combustion dynamics. This study proposes a multiple-relaxation-time (MRT) lattice Boltzmann (LB) method to simulate solid coke combustion in porous media at the pore scale. The MRT LB model can enhance numerical stability compared with the single-relaxation-time counterpart. This new LB model offers two major advances over existing LB models. First, thermal expansion effects are considered via temperature-dependent density, satisfying the low Mach number flow condition. Second, conjugate heat transfer and species conservation at the coke-fluid interface are modelled by a source term and a bounce-back scheme, respectively, without resorting to iterative methods. Simulations of pore-scale coke combustion are conducted to validate the superior performance of the developed LB model. The results show that this model can reproduce coke combustion dynamics and capture variations in isotherms from the fluid phase to the solid phase at the pore scale. In contrast, without considering effects of thermal expansion or conjugate heat transfer as in previous models, simulations would underestimate the burning temperature and even fail to predict instability caused by a high driving force. Moreover, using the proposed LB model, a parametric study is performed to assess the impact of the inlet air and porous structure on coke combustion. The findings suggest that inlet air temperature and driving force should be controlled within certain ranges, otherwise their influences may become negligible or even negative. Large porosity accelerates coke combustion and leads to a desired combustion front. Heterogeneous coke distribution affects pore-scale combustion details but not the overall combustion intensity, while a random matrix structure may influence combustion dynamics both locally and globally. As a whole, this study develops a reliable LB model to investigate pore-scale coke combustion, which contributes to advancing the knowledge base for heavy oil recovery using the ISC process.

Two-Dimensional Numerical Study of Methane-Air Combustion Within Catalytic and Non-catalytic Porous Medium.

This study numerically investigates a two-dimensional physical model of methane/air mixture combustion in catalytic and non-catalytic porous media. The temperature distribution and flame stability of combustion in inert alumina (Al2O3) pellets and platinum (Pt) catalyst-supported alumina (Al2O3) pellets, were studied by changing the burner structure, operating parameters, and physical properties of alumina pellets. The simulation results indicated that the gas temperature in the inert porous medium is higher than that in a catalytic porous medium, while the solid temperature in an inert porous medium is lower than that in a catalytic porous medium. The flame moved toward the burner exit with the increasing diameter of the packed pellets at a lower equivalence ratio and moved toward upstream with the increased thermal conductivity of packed pellets. The flame location of the catalytic porous burner was more sensitive to the flame velocity and insensitive to thermal conductivity compared to the inert porous burner. The distance of the flame location to the burner inlet is almost constant with the increasing length of the porous media for both the catalytic and inert porous burner, while the relative position of the flame location moved toward the upstream.

Can Small Polyaromatics Describe Their Larger Counterparts for Local Reactions? A Computational Study on the H-Abstraction Reaction by an H-Atom from Polyaromatics.

Hydrogen abstraction is one of the crucial initial key steps in the combustion of polycyclic aromatic hydrocarbons. For an accurate theoretical prediction of heterogeneous combustion processes, larger systems need to be treated as compared to pure gas phase reactions. We address here the question on how transferable activation and reaction energies computed for small molecular models are to larger polyaromatics. The approximate transferability of energy contributions is a key assumption for multiscale modeling approaches. To identify efficient levels of accuracy, we start with accurate coupled-cluster and density functional theory (DFT) calculations for different sizes of polyaromatics. More approximate methods as the reactive force-field ReaxFF and the extended semi-empirical tight binding (xTB) methods are then benchmarked against these data sets in terms of reaction energies and equilibrium geometries. Furthermore, we analyze the role of bond-breaking and relaxation energies, vibrational contributions, and post-Hartree-Fock correlation corrections on the reaction, and for the activation energies, we analyze the validity of the Bell-Evans-Polanyi and Hammond principles. First, we find good transferability for this process and that the predictivity of small models at high theoretical levels is way superior than any approximate method can deliver. Second, ReaxFF can serve as a qualitative exploration method, whereas GFN2-xTB in combination with GFN1-xTB appears as a favorable tool to bridge between DFT and ReaxFF so that we propose a multimethod scheme with employing ReaxFF, GFN1/GFN2-xTB, DFT, and coupled cluster to cope effectively with such a complex reactive system.

Effect of combustion zone material on the thermal performance of a biogas-fuelled porous media burner: Experimental studies

The performance of a porous media burner (PMB) is immensely dependent on the selection of porous material. Present experimental work investigates the impact of burner’s material on the thermal performance of a crude biogas-fuelled, in house developed PMB. This includes the study of burner stable operating limits, location of reaction zone, estimation of radiation efficiency and exhaust gas emission analysis. The PMB consists of two sections of porous material, one is the combustion zone and the other is the preheat zone. In the present research, the impact of combustion zone material on performance of the PMB is tested, while keeping the material of preheat zone unchanged. Silicon carbide (SiC) and zirconia (ZrO2) foams, both having porosity of 10 ppi, have been chosen as combustion zone. Experimental investigation shows that for the whole input load range of 5–10 kW, the SiC foam offered lean working condition, while in case of ZrO2, similar condition was limited to 5–8 kW. Within these stable operating limits, the axial temperature mapping of the burner highlights the location of reaction zone. Combustion in SiC foam is found to offer higher radiation efficiency of maximum 23.7%, as compared to that of ZrO2, where it is only 8%. Exhaust gas analysis shows that CO and NOx concentrations of both burners were low and the maximum values were limited to 156 ppm and 20 ppm, respectively. Overall performance assessment confirms that SiC is a better choice over ZrO2 for lean combustion of crude biogas in the developed PMB.

Study on Combustion Characteristics of Premixed Methane-Oxygen in a Cylindrical Porous Media Combustor

A three-dimensional numerical and experimental study on methane-oxygen combustion has been conducted in a small scale porous-media cylindrical combustor which aims at extending the optimized controlling parameters for thermophotovoltaic power application. Different equivalence ratios, inlet velocities, porous-media porosities and wall thermal conductivities are simulated in the same configuration and the results show that porous-media improves the thermal characteristics of a combustor compared to the non-porous-media counterpart. There is about 350 K difference in peak wall temperature for the porous-media combustor over the non-porous-media combustor. The flame temperature first increases as porosity increases, and then slightly decreases as the porosity increases. At higher wall thermal conductivities, the effect of the change in porosity does not affect the mean temperature distribution on the combustor wall. Based on the findings, the porous-media combustor with equivalence ratio of 0.8, porosity of 0.5, inlet velocity of 0.35 m/s and wall thermal conductivity of 20 W/(m2219;K) are identified as key combustion conditions that can produce uniform and stable wall temperature for an efficient combustion chamber for the thermophotovoltaic system. With the inherent constraint associated with small-scale combustion this study will provide a premise for developing and enhancing the performance of the combustor under the given constraints.

Heterogeneous and platinum-initiated homogeneous combustion of nitrogen-containing heterocyclic compounds in the argon and water vapor media at elevated pressure

The paper presents the results on the oxidation of pyrrole and pyridine in the argon and water vapor media at high pressure and uniform heating of the tubular stainless steel reactor to 823 K. Experiments were carried out with lean and stoichiometric mixtures (fuel equivalence ratio φ = 0.78–1.01) at their dilution level x D = 36–65% mol. Based on the time dependences of the reaction mixture temperature, it is revealed that oxidation proceeds via the mechanisms of homogeneous and heterogeneous reactions. It was found that the oxidation rate of pyrrole depends on the amount of corrosion products accumulated on the reactor wall. It is shown that a Pt-Rh/Pt thermocouple inserted into the center of the reaction volume intensifies pyridine oxidation in the near-surface region. The difference in the oxidation mechanisms of pyrrole and pyridine is explained by the structure of their molecules.

Stability study of hydrogen-air flame in a conical porous burner

Porous medium combustion offers several advantages such as extended flammability limits, high burning rates and low pollutant emissions. In this work, laminar premixed flame of hydrogen/air mixture in a conical porous burner is numerically investigated by using OpenFOAM CFD toolbox. An unsteady, two-dimensional mathematical model is constructed. Flame front location within the burner is not predetermined in this model. A baseline case is first investigated using detailed kinetics for hydrogen combustion. Flame stability analysis is then performed and the effect of the burner cone angle on flame location is studied. For this purpose, four different burner cone angles with seven different firing rates are examined. It is observed that the burner cone angle has no considerable effect on flashback and blowout limits. Variation of the flame location in the porous medium with the burner cone angle and firing rate is also investigated. It is concluded that increasing the firing rate for a fixed burner cone angle, moves the flame location downstream in the burner and also increases the length of the preheating zone. On the other hand, an increase in the burner cone angle at a fixed firing rate causes the flame to move upstream and also shortens the preheating zone length.

Ash Fusion During Combustion of Single Corn Straw Pellets

AbstractThis investigation identified conditions at which corn straw ash melts and examined how this phenomenon affects the combustion of char residues. Corn straw was pelletized in cylinders and was burned at elevated temperatures in the range of 1200–1400 °C, and at different air flow velocities. The pellets were inserted in a preheated furnace, where they were subjected to moderately high heating rates. Their combustion behavior was observed with cinematography, thermometry, and thermogravimetry. Upon insertion in the furnace, the pellets devolatilized and formed volatile envelope flames, upon extinction of which, the chars experienced concurrent heterogeneous combustion and ash fusion. Residues were assessed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), coupled to energy dispersive spectroscopy (EDS). The burnout times of the volatiles and the chars decreased drastically with increasing temperature. At 1300 °C and 1400 °C, the remaining ash underwent complete melting and the final structures of the pellets collapsed to molten pools. At 1400 °C, all of the chlorine and most of the potassium were released into the gas phase. The straw ash was identified as a high-density silicate melt. Although surface ash melted completely, it flowed to the base of the pellet. Therefore, it did not significantly hinder the oxidation of the carbonaceous char. Hence, to increase the likelihood of complete corn straw carbon burnout and of ash melting and flowing to the bottom of the furnace, operating temperatures higher than 1300 °C, in conjunction with mild air flow rates, are recommended.

A tabulated chemistry method for heterogeneous solid propellant combustion

Detailed chemistry simulations of heterogeneous solid propellant combustion at the microstructure scale are very costly as they involve large and stiff chemical schemes in the gas phase. A usual workaround is to use global chemistry to approximate gas phase reactions. A drawback of this approach is the need for an a priori calibration which necessitates reference data and often limits the operating conditions range of validity. An alternative method based on 1-D burner-stabilized flames tabulation is proposed in this paper. The theoretical and numerical frameworks are detailed. An appropriate tabulation space is introduced to store useful thermochemical variables as well as Jacobian terms used for time-integration. Jump conditions are also rewritten to cope with tabulated chemistry formalism. Several 1-D validations are performed on classical propellant compositions. Detailed comparisons with available experimental data demonstrate the ability of the proposed methodology to tackle inner flame structure (temperature and species) as well as the evolution of the burning rate with pressure and composition. Finally, 2-D laminate propellant simulations are conducted showing the ability of the methodology to capture complex flame regimes and associated burning rate modifications.

Data driven investigation of local combustion mode in heterogeneous combustion

Data driven investigation of local combustion mode in heterogeneous combustion

3D numerical study on syngas production in a structured packed bed with connected pellets

Fuel-rich partial combustion in porous media is numerically studied at the pore scale with 3D staggered arrangement of connected pellets. The chemistry is treated with detailed chemical mechanism and the solid-to-solid radiation is taken into account. The numerical results are validated against experimental results and indicates that 3D pore scale numerical modeling gives reasonable syngas components. Considerable heat recirculation in the preheating zone is predicted by 3D pore-scale model. Root mean squares (RMS) is used to quantitatively estimate the local spatial variations of the velocity and gas temperature. It is shown that the distribution of RMS of gas temperature along flow direction is basically wave-like shape and the spatial variation of gas velocity reaches 78% of the average velocity.

Theoretical analysis of superadiabatic combustion for non-stationary filtration combustion by excess enthalpy function.

The superadiabatic combustion for non-stationary filtration combustion is analytically studied. The non-dimensional excess enthalpy function (H) equation is theoretically derived based on a one-dimensional, two-temperature model. In contrast to the H equation for the stationary filtration combustion, a new term, which takes into account the effect of non-dimensional combustion wave speed, is included in the H equation for transient filtration combustion. The governing equations with boundary conditions are solved by commercial software Fluent. The predictions show that the maximum non-dimensional gas and solid temperatures in the flame zone are greater than 3 for equivalence ratio of 0.15. An examination of the four source terms in the H equation indicates that the thermal conductivity ratio between the solid and gas phases is the dominant one among the four terms and basically determines H distribution. For lean premixed combustion in porous media, the superadiabatic combustion effect is more pronounced for the lower .