Heterogeneous Combustion Research Articles

Sustainable hydrogen production through coupling isopropanol-butanol-ethanol with enhanced heat recirculation

The utilization of isopropanol-butanol-ethanol (IBE) to clean energy effectively mitigated the problems of environmental pollution and sustainable energy regeneration. In order to improve the utilization efficiency of ethanol and isopropanol in IBE, a liquid double-layer porous media burner was designed. And the combustion characteristics with different porous pellets and isopropanol ratios were studied. The results indicated that the mole fraction of hydrogen decreased with the increasing pellet diameter, and the lower heating value of syngas reached the highest value of 82.0 MJ/Nm3 when the pellets possessed two holes. Meanwhile, the peak temperature reached the maximum of 1009 K when the water content was 20% due to the sufficient combustion. And the isopropanol was conducive to the improvement of the total energy conversion efficiency. In addition, the yields of hydrogen and carbon monoxide were improved by 23.7% and 30.5% when the velocity increased from 10 to 12 cm/s. These results showed that the combustion of ethanol and isopropanol in porous media burners achieved the sustainable production of hydrogen and efficient utilization of IBE biofuels.

Stretching and Compression of the Quasi-Isobaric Filtration Front of Combustion of Porous Media

Stretching and Compression of the Quasi-Isobaric Filtration Front of Combustion of Porous Media

Experimental and theoretical study of single iron particle combustion under low-oxygen dilution conditions

In the present study, a novel in situ particle sizing approach is proposed and used to measure the characteristic timescales of micron-sized iron particle combustion under low-oxygen (10–17vol%) dilution conditions. The particle size is determined by probing the light emission intensity of a burning particle during melting, which is proportional to the cross-section area of the particle projected to the camera. Detailed descriptions of the calibration, validation, and characterization of the experimental method are elaborated. With systematic measurements, we obtain one-to-one correlations between combustion timescales and single particle diameters at various diluted oxygen concentrations. Furthermore, we formally derive a theoretical model for heterogeneous combustion of growing (iron) particles in the diffusion-limited regime. The model suggests that the diffusion-limited burn time scales with the initial particle diameter squared (i.e., a new, generalized d2-law). Owing to accounting for the particle growth, the newly derived model suggests a significantly (1.66 times) shorter combustion duration compared to the conventional d2-law for shrinking particle combustion. It turns out that the new model agrees well with the experiment. This agreement also suggests that under low-oxygen dilution conditions, the combustion regime of iron particles during the intensive burning stage (i.e., from ignition to the peak particle temperature) is limited by external oxygen diffusion.

Experimental investigation of the co-combustion of LPG-hydrogen blends on LPG-fueled systems

The need for sustainable and less pollutant heating solutions has started an energy transition process. Some of the short-term alternatives proposed are based on the use of hydrocarbon-hydrogen blends in conventional combustion systems. In this work, LPG-H2 blends are studied in (a) a gas heater, (b) a water heater and (c) a porous media burner. Thermal and emission behavior, as well as thermal efficiency, are studied for different fractions of H2 in the fuel blend. Stable combustion was sustained for H2 volume fractions up to (a) 44%, (b) 68% and (c) 58%, for the above-mentioned appliances, respectively. Both gas and water heaters showed an overall reduction in thermal efficiency and temperatures as H2 presence was increased. On the other hand, higher temperatures, and thermal efficiencies, along with a 30% reduction in CO emissions were registered for the porous media burner for large H2 fractions in the fuel blend. In conclusion, the technical feasibility of adding H2 to the LPG feed lines of conventional combustion systems was demonstrated, through the stable operation of the 3 appliances studied.

Effect of particle size distribution on the laminar flame speed of iron aerosols

Iron particles in air burn in heterogeneous combustion mode and the flame speed is very sensitive to particle size. Previous numerical work on the propagation of flames in iron dust was based on average particle sizes. However, in practice, experiments are conducted with particle size distributions (PSD). This makes it challenging to compare different experiments as the samples used in those studies vary (the average particle size might be similar but not the particle size distribution). It is the aim of the current work to provide insight into the effect of particle size distribution on flame propagation. This involves identifying the minimum number of discrete averaged particle sizes (bins) required in the simulations to capture the burning characteristics of the PSD. Then, flame speed and flame structure for a narrow and broad distribution are investigated. It is shown that the equivalence ratio at which the maximum flame speed occurs for certain mean particle sizes varies with the width of the particle size distribution. The difference in the flame speed between the same average particle size but different standard deviation varies as a function of β (ratio between standard deviation and average particle size), not just the average particle size itself. For a constant β at a particular equivalence ratio, the difference in the flame speed between PSD and mono-dispersed aerosols is approximately the same irrespective of the particle size. The effects of the smaller and bigger particles in the PSD on the flame speed and flame structure are also systematically investigated. The findings in this study confirm that the particle width of the PSD plays a crucial role and that experiments and simulations can not be readily compared if different PSDs are used.

Improving volume-averaged simulations of matrix-stabilized combustion through direct X-ray µCT characterization: Application to NH3/H2-air combustion

Porous media combustion (PMC) relies on internal heat recirculation in an open-cell ceramic foam matrix to enhance the flame speed of fuels with poor combustion properties. Volume-averaged simulations are often used to study the combustion performance and pollutant emissions of such systems. However, due to the varying complexity of matrix geometries found in practical burners, as well as the wide range of closure models for the constitutive relations of the solid phase, contradicting statements about the predictive accuracy of these volume-averaged models can be found in the literature. In this work, we propose an open-source modeling framework for accurate volume-averaged PMC simulations by using first-principles methods to determine effective properties used in closure models. This framework relies on adequately characterizing the topology of the solid matrix, using commonly available X-ray computed microtomography. With this approach, significant improvements in accuracy are reported compared to empirical models from the literature. The framework based on first-principle evaluations of constitutive relations is compared against experimental measurements conducted on an interface-stabilized burner operated with premixed NH3/H2-air. The model shows good agreement for exhaust gas composition and stability limits. The proposed simulation framework performs significantly better than state-of-the-art techniques that employ commonly used empirical correlations for effective matrix properties. Statement of SignificanceWe present a new open-source simulation framework for improved characterization of porous media combustion. By utilizing µCT techniques, accurate effective matrix properties can be determined from first-principle simulations. These effective properties are used in closure models for 1D volume-averaged reacting flow simulations using appropriate sub-models for heat recirculation. This modeling framework is able to reliably predict stability limits while conventional closure models yield erroneous trends. Assessment of the resulting modeling framework is performed using experiments with exhaust gas characterization performed on a NH3/H2-air porous media burner.

Multi-objective optimization study of hydrothermal aging on NOx-assisted soot catalytic combustion and secondary pollution emission in regeneration of CeO2 catalytic diesel particulate filter

The present study aimed to explore the effect of hydrothermal aging on the heterogeneous catalytic combustion process of soot in CeO2 catalytic diesel particular filter and the formation of secondary pollutants (N2O and CO).The present study was based on a fixed-bed experimental bench for NOx-assisted soot catalytic oxidation in CeO2-CDPF with different soot loading and catalyst coating amounts. A non-linear regression surrogate model and a multi-objective optimization decision approach were used to analyze the trade-off relationship between the design variables and the optimization objectives. The results showed that lower levels of soot loading (2.5 g/L), CeO2 catalyst coating amount (2.5 g/L) and higher NOx concentration (1000 ppm) in fresh catalysts were favorable for improving the performance of soot-assisted catalytic combustion and minimizing the formation of N2O and CO. However, it was observed that higher levels of catalyst coating amount (10 g/L) resulted in better soot catalytic combustion performance with lower N2O and CO emissions. Global sensitivity analysis showed a positive correlation between N2O and CO peak concentrations and peak temperatures with soot loading, but a negative correlation with catalyst loading. Over time, hydrothermal aging of CDPF caused the inhibitory effect of CeO2 catalyst on N2O to shift from negative to positive correlation, while also decreasing the synergistic performance of NOx in soot catalytic combustion.

Properties and mechanisms of low concentration methane catalytic combustion in porous media supported with transition metal oxides

Low concentration methane (LCM) from coal mining is hardly utilized by combustion or oxidation due to its low concentration and fluctuation. Herein, a porous media catalytic combustion technology was presented to investigate the catalytic combustion properties of LCM in porous media by developing three porous media catalysts (Fe2O3/Al2O3, CuO/Al2O3 and Co3O4/Al2O3) supported with transition metal oxides (Fe2O3, CuO and Co3O4) as active components. While the catalytic combustion mechanism was proposed by density functional theory (DFT) calculations. The results indicated that Fe2O3/Al2O3 exhibited superior high-temperature catalytic activity and thermal stability, which was capable of broadening the limit equivalence ratio of CH4 stationary combustion to 0.46 with the CH4 conversion rate over 98 %. DFT calculations revealed that the Fe atoms on the (001) surface of Fe2O3 crystals were the active center during the catalytic process, causing the adsorption and dissociation of CH4 and O2 molecules. The lattice oxygen of Fe2O3 was engaged in the formation of CO and CO2 and the adsorbed oxygen from O2 molecules dissociation facilitated H2O generation, which heightened the conversion of CH4. This study provides a promising avenue for the high-efficiency and clean utilization of LCM.

Experimental Investigation of a Self-Sustained Liquid Fuel Burner Using Inert Porous Media

A self-sustained porous burner without a sprayed atomizer was built for diesel oil. It consisted of metal fiber felt as an evaporator upstream and ceramic foam as an emitter downstream. The liquid fuel underwent film boiling in the porous evaporator and was rapidly evaporated by the heat recirculated from the porous emitter to the porous evaporator through intense irradiative heat flux. The effect of the porous structure and its installation location on the performance of the porous burner was investigated. The results indicated that the evaporation and combustion of liquid fuel could be prompted by the radiation of porous media. The position of the flame moved downstream, and the flame temperature decreased when the distance between the metal fiber felt and the ceramic foam was increased. The lowest NOx concentration was obtained when the distance between the foam and the metal fiber felt was 90 mm. When the diameter of the central hole of the ceramic foam was increased, the position of the flame moved towards the burner outlet, and the flame temperature and NOx emission declined. The flame temperature of the divergent configuration as emitter was higher than that of the convergent configuration, and the flame temperature of the C–D configuration was higher than that of the D–C configuration. Different ceramic foam structures had a significant effect on the temperature and emission in the combustion chamber, which showed that the evaporation and radiation performance of inert porous media burners with different structures is quite different.

Study on Oxy-Methane Flame Stability in a Cylindrical Porous Medium Burner

Combustion in a porous medium can be beneficial for enhancing reaction rate and temperature uniformity. Therefore, considering the combination with oxy-fuel combustion can address some shortcomings in oxy-fuel burners, a cylindrical two-layer porous burner model is established based on OpenFOAM in this paper. A two-temperature equation model is adopted for the simulation of the heat transfer process. The CH4 skeletal kinetic mechanism is adopted for complex chemistry integration based on OpenSMOKE++. Corresponding experimental methods were used for complementary studies. The walls of the burner are wrapped with three types of thermal insulation materials to present different levels of heat loss. The results show that considering the convection and radiative heat loss of the burner wall, the temperature near the wall is reduced by more than 300 K compared to the adiabatic condition. As a result, the flame propagation speed and CO oxidation rate slowed down. The stable range will be destructively narrowed by more than 50%, and CO emissions will increase by more than 10 times. These defects will be aggravated by increasing the diameter of the burner. It is observed that when the diameter of the burner increases from the initial 5 cm to 10 cm, the effect of heat loss on the stable range is insignificant.

Co-partial oxidation of acetic acid, methanol, and ethanol in an integrated porous media with efficient heat recirculation: Green hydrogen production and its expansion method

Green energy from biomass contributed to substituting the traditional fossil fuels and environmental protection. To explore the hydrogen production from bioethanol with the industrial by-products, a porous media burner integrated with a heat-recirculating tube was designed to realize the efficient vaporization of ethanol. Then the partial oxidation characteristics of ethanol were investigated with the acetic acid and methanol addition at different operating conditions. The results indicated that the acetic acid and methanol addition inhibited hydrogen production but promoted methane production. Compared with pure ethanol, the heating values of syngas were decreased by 3.41 and 0.64 MJ/Nm3 with the acetic acid or methanol content of 20%. The highest yields of hydrogen and carbon monoxide were achieved at 43.94% and 66.45% respectively when the ratio of acetic acid content to methanol was 5:15. Nevertheless, blended fuels achieved more syngas production at higher equivalence ratios and velocities. The hydrogen concentration of the blended fuel reached the maximum of 21%, exceeding that of pure ethanol by 1.6% when the velocity was 22 cm/s. The corresponding results provided a theoretical reference for the industrial application of bioethanol.

Combustion temperature in a three-dimensional porous stove with a high-fidelity structure

The combustion temperature in a three-dimensional (3-D) porous stove was investigated numerically by using a novel high-fidelity structure of ceramic foam. The surface temperatures of the present stove were compared with those in corresponding experiments to verify the numerical model. Based on the discussion of gas velocity in the combustion state, the internal combustion temperature in the 3-D structure was analyzed by focusing on the solid–gas temperature profiles and the heat transfer characteristics. The results showed that large Venturi effects occurred with formation of jets on the upper surface of the ceramic foam. Numerous discrete flames were clearly formed in the 3-D pores of the ceramic foam and not only showed wrinkling induced by the staggered skeletons, but also differed significantly owing to the difference in pore structures, resulting in different temperature profiles in different axial cross-sections. The temperature for the axial cross-section with the same height was basically same in the circumferential direction, with even the same temperature difference between gas and solid. The driving force of internal heat transfer in porous media combustion (PMC) originates from the gas–solid temperature difference, resulting in the formation of an enclosing steady self-heat circulation process. The surface heat transfer coefficient (SHTC) obtained in this work agreed well with those of corresponding experiments in previous work. Their maximum region emerged near the interface between the distributor and foam ceramic, and not in the region of the maximum combustion temperature. The heat transfer coefficients gradually decreased along the axial direction in spite of undergoing combustion expansion, whereas it increased from the center to the outside edge in the radial direction. The results are conducive to numerical simulation of PMC, and helpful for combustion in domestic gas stoves.

Unsteady heat and mass transfer mechanism in AP/HTPB propellant combustion under rapid depressurization

This work is to investigate the dynamic instability and unsteady heat and mass transfer mechanism of Ammonium perchlorate/Hydroxyl-terminated polybutadiene (AP/HTPB) propellant in the chamber of solid rocket motor (SRM). The environment of exponential depressurization is imposed on the heterogeneous propellant combustion to simulate the actual working condition of SRM. The microstructure of AP/HTPB propellant is generated by a code based on the sequential algorithm. The combustion model consists of gas reaction, heat conduction in the solid phase, and the non-planar moving surface. A quenched model under the rapid pressure decay is established. It is found that the rapid depressurization and the change of surface morphology are responsible for the combustion fluctuation. Finally, quenched times of different propellant packs are calculated. It is found that different AP particle positions in the propellant pack will also cause a change in the quenched time.

Visual investigation on optimizing combustion characteristic of porous media burner combined with bluff body

ABSTRACT Coalbed methane as an unconventional natural gas is suitable for the utilization in thermal photovoltaic systems through optimized burners. In order to improve the radiation and utilization efficiency of coalbed methane, a combination of bluff body and porous media was proposed. Moreover, the bluff body spacing and porous media structure were changed to analyze the influence on flame characteristics. The results showed that the maximum temperature and radiation efficiency increased with decreasing of bluff body spacing. When the bluff body spacing was 30 mm, the maximum temperature of 933 K was obtained, and the radiation efficiency increased by 68%. The increase of the equivalence ratio also improved the radiation efficiency, and the maximum radiation efficiency was 14.5% at φ = 0.90. In addition, the increase of inlet velocity and bluff body spacing was beneficial to downstream flame propagation, but had a negative effect on upstream propagation. At v in = 40 cm/s, the maximum downstream propagation velocity was 0.267 mm/s. Due to the difference in thermal conductivity, flame fragmentation occurred in the 8 mm pellets, but the flame propagation was gradually stable with the increase of bluff body temperature.

Modern Competing Flames Model for Composite Ammonium Perchlorate/Hydroxyl-Terminated Polybutadiene Propellant Combustion

The competing flames model, also termed the Beckstead–Derr–Price model, for steady-state heterogeneous propellant combustion has been widely used but has not been sufficiently updated in decades or compared to modern propellant combustion databases. In the current study, historical competing flames modeling approaches were thoroughly documented; and an improved framework was outlined and updated to include several improvements, such as variable flame temperatures, specific heat capacities, and latent heat terms. Model parameters were initially taken from previous literature, but the fuel and diffusion flame parameters were optimized based on a compiled database of unimodal propellant burning rates from the literature spanning a wide range of ammonium perchlorate (AP) particle sizes (), AP mass concentrations (70–87.5%), and combustion pressures (0.7–20.7 MPa). The improved model was compared to AP monopropellant, unimodal, and multimodal propellant burning rate databases from the literature. General dependencies of the burning rate-to-oxidizer concentration and size were accurately captured. The predictive capability of the improved model for AP monopropellant burning rates and unimodal propellant formulations was excellent, where the only significant discrepancies were noted for very fine AP particles (). Model predictions for multimodal formulations were moderate and could be improved by alternative pseudopropellant apportionment and statistical accounting schemes.

Assessment of indoor air quality of porous media combustion-based cookstoves.

The present study analyzes the emission mitigation ability of a porous media combustion (PMC) technology-based cookstove compared to a free flame combustion (FFC) technology-based cookstove. Emission of pollutants, i.e., PM2.5, PM10, and CO, caused due to burning of fuels, namely, methanol, ethanol, kerosene, and LPG in the kitchen environment are measured. The study incorporated exhaustive real-time indoor air quality (IAQ) measurements and presented the temporal variation of measured pollutant concentrations for 2 h (morning meal duration). In addition, 24 h average concentration of the measured pollutants is also compared with the limits prescribed in WHO guidelines for domestic settings. The results emphasized that the utilization of cookstove based on PMC would help in improving the IAQ of the kitchen area by decreasing the concentrations of PM2.5, PM10, and CO. For 2 h duration measurements, the methanol cookstove based on PMC reduced the concentrations of PM2.5, PM10, and CO by 7.7%, 8.1%, and 17.2%, respectively, compared to FFC cookstove. Similarly, in the case of PMC-based LPG cookstove and kerosene cookstove, the respective values were 11.7%, 20.4%, and 41.6% and 55.3%, 62.6%, and 66.6%. Among all the tested cookstoves, PMC-based LPG cookstove achieved the lowest emission values (PM2.5: 20.6 μg/m3, PM10: 31.3 μg/m3, and CO: 1 ppm) which are lower than the prescribed WHO values (PM2.5: 25 μg/m3, PM10: 50 μg/m3, and CO: 6 ppm).

Toward pore-scale simulation of combustion in porous media using a low-Mach hybrid lattice Boltzmann/finite-difference solver

A hybrid numerical model previously developed for combustion simulations is extended in this article to describe flame propagation and stabilization in porous media. The model, with a special focus on flame/wall interaction processes, is validated via corresponding benchmarks involving flame propagation in channels with both adiabatic and constant-temperature walls. Simulations with different channel widths show that the model can correctly capture the changes in flame shape and propagation speed as well as the dead zone and quenching limit, as found in channels with cold walls. The model is further assessed considering a pseudo two-dimensional porous burner involving an array of cylindrical obstacles at constant temperature, investigated in a companion experimental study. Furthermore, the model is used to simulate pore-scale flame dynamics in a randomly generated three-dimensional porous media. Results are promising, opening the door for future simulations of flame propagation in realistic porous media.

Effect of hetero-/homogenous combustion on energy conversion performances and flame stability of methane/air-fueled micro-combustors with heat-recirculating structure and platinum-coated

In this work, the joint effect of applying Pt catalyst and heat-recirculating structure on the flame stability and thermal performance has been numerically investigated in methane-air-fueled micro-combustors by a detailed three-dimensional computational model. Numerical results indicate that such strategies contribute to improving outer wall temperature by 14.4% and extending the blowout limit by a factor of 0.6 compared to those in the straight non-catalytic combustor. Such enhancements are due to the intensified heat transfer, thus highlighting the contributions of heterogeneous combustion chemistry and heat-recirculating effect. Homogeneous chemistry in the presence of a Pt catalyst is shown to be self-sustained over wide-ranging conditions. Meanwhile, the effectiveness of heat recirculation structure on combustion performance is more significant as compared to that of the catalytic combustion effect, as evidenced by a 4% and 17.6% improvement in the wall mean temperature and blowout limit respectively. Further analysis of the heat recirculation catalytic combustors is conducted to obtain the optimal geometry. There exists a critical flow rate corresponding to the maximum outer wall temperature and radiation energy. In general, this work demonstrates the viability of applying a Pt catalyst and heat-recirculating structure to extend the flame stability limit and energy conversion performance in micro-power systems.

Ignition and combustion of a group of waste-derived fuel slurry droplets: Mutual influence upon low-temperature convective heating

Waste-derived slurries can be burned both in a fluidized bed and when injected into a boiler furnace. This paper is the first to analyze the dynamic characteristics of ignition and combustion of a group of three slurry droplets based on coal slime, water, and waste industrial oil in a low-temperature air flow with a temperature of 500–600 °C and velocity of 0.1–1.2 m/s. The distance between the droplet centers varied from 3 to 15 mm. The results of the study can develop innovative technologies for flare and fluidized bed combustion of fuel slurries at the boiler start-up stage. It was found that a distance of 3–5 mm between droplet centers provides almost simultaneous gas-phase ignition of all droplets. A further increase in the distance critically worsened the ignition and combustion and eliminated the synergy. An increase in the air flow velocity contributed to the appearance of a common flame zone but at the same time increased the ignition delay of droplets by a factor of 1.2–1.6. The negative effects of cooling and entrainment of combustion products by a low-temperature flow can be effectively reduced by adding 5 wt% of industrial oil to the slurry. Also, the use of oil reduced the droplet ignition delay time by 15–20%. At the stages of flame and heterogeneous combustion, the differences for droplets in a group increased (the stage durations differed by 1.5–3 times). For the low-temperature start-up of a boiler, it was found advisable to provide a droplet flow with a sufficient density to stabilize their ignition without significant flow deviations (dimensionless distance 1.5–2.5).

Experimental investigation on the partial oxidation of methane with the effect of shrunk structure

Hydrogen energy is an ideal clean energy to solve the expanding energy demand and environmental problems caused by fossil fuels. In order to produce hydrogen, a double-layer porous media burner with shrunk structure was designed to explore the partial oxidation (POX) of methane. And the combustion temperature, species concentration and reforming efficiency were studied under different shrunk parameters and operating conditions. The results indicated that the shrunk structure greatly influenced the flame position and temperature distribution. The flame moved to the downstream section with the decreasing of the inner shrunk diameter and the increasing of the shrunk height. When the diameter of the filled Al2O3 pellets was 8 mm, the hydrogen yield reached the highest value of 43.8%. With the increasing of equivalence ratio, the reforming efficiency increased first and then decreased, and the maximum value of 53.0% was reached at φ = 1.5. However, the reforming efficiency and axial temperature kept increasing when the inlet velocity increased from 10 to 18 cm/s. The corresponding results provided theoretical reference for the control of flame position and species production by the design of shrunk structure in porous media burner.