Investigation of the structural and reactants properties on the thermal characteristics of a premixed porous burner

Porous burners offer attractive features such as competitive combustion efficiency, high power ranges, and lower pollutant emissions. In the present study, the thermal characteristics of a porous burner are numerically investigated for a range of operating conditions and design specifications within a practical range. The premixed flame propagation of a methane/air mixture in a ceramic porous medium is simulated through an unsteady, one-dimensional model. The combustion process is modeled using a suitable single-step chemical kinetics. The reaction location is not predetermined, thus the flame is allowed to float within the solid matrix or to run off from either side of the porous medium. The numerical results indicate that flame stability and thermal characteristics of the burner are strongly dependent on the inlet mixture specifications and the solid matrix structural properties. For a fixed value of the inlet firing rate, the combustion products temperature will increase by an increase in the inlet gas temperature, an increase in the matrix porosity, or by a decrease of the matrix pore density. Among the geometrical properties, the burner length has virtually no effect on the burner performance. An increase in the solid matrix porosity or burner firing rate will increase the efficiency of the preheating zone, while increasing the inlet gas temperature or matrix pore density will cause a reduction in this efficiency. Simulation results also suggest that in order to prevent flame blow-out or flash-back, critical values of the burner settings and design parameters must be avoided.     

有序堆积床内预混气体燃烧特性实验研究

为研究预混气体在多孔介质燃烧器中的火焰燃烧特性,设计了一种新型多孔介质燃烧器,其中多孔介质区域由氧化铝圆柱体有序堆积而成.分别研究了当量比和入口速度对甲烷/空气预混气体在多孔介质燃烧器中的火焰温度分布、火焰最高温度以及火焰传播速度的影响.结果 表明:在当量比0.162～0.324、入口速度0.287～0.860 m/s...     

Experimental investigation of combustion in porous heating burners

The combustion characteristics of liquefied petroleum gas inside porous heating burners have been investigated experimentally under steady-state and transient conditions. Cooling tubes were embedded in the postflame region of the packed bed of a porous heating burner. The flame speed, temperature profile, and [NO_x] and [CO] in the product gases were monitored during an experiment. Due to the heat removal by the cooling tubes, a phenomenon termed metastable combustion was observed; this is that only one flame speed exists at a particular equivalence ratio for maintaining stable combustion within the porous bed of the porous heating burner. This behavior is quite different from that of porous burners without cooling tubes, in which an extended range of flame speeds usually is found for maintaining stable combustion. After metastable combustion has been established in a porous heating burner, a change in the equivalence ratio will stop the metastable combustion and drive the flame out of the packed bed. From the steady-state results, the porous heating burner was shown to maintain stable combustion under fuel-lean conditions with an equivalence ratio lower than the flammability limit of a normal free-burning system. The flame speed in a porous heating burner was found to decrease with an increase in the length of the porous bed. Combustion within a porous heating burner has the features of low flame temperature, extended reaction zone, high preheating temperature and low emissions of NO_x and CO. The flame temperature ranged from 1050 to 1250 °C, which is ∼200 °C lower than the adiabatic flame temperature at the corresponding equivalence ratio. The length of the reaction zone could be more than 70 mm and the preheating temperature ranged from 950 to 1000 °C. Both [NO_x] and [CO] were low, typically below 10 ppm.     

Approximate solutions of lean premixed combustion in porous media with reciprocating flow

Based on the analogy with the steady countercurrent reactor, a simplified theoretical solution is presented, which is applicable to adiabatic inert porous media combustors with reciprocating flow. The model consists of two ordinary differential equations that link all major controlling parameters, which allow for a good physical understanding of the process. The maximum temperatures in the burner predicted by the simplified model show the same trends as those in experimental results, but are generally higher, and the discrepancy between the experimental data and predications is less than 20%. By analyzing experimental and simulation results, a simplified theoretical solution for the temperature profile in the burner is further developed, which is expressed in terms of a piecewise linear function and the lean flammability limit is presented by a implicit expression. Results show that the lean flammability limit can be extended by using porous media of smaller pore size. The predicted lean flammability limit provides guidelines for the design of the combustor and some indications for further improving the combustor performances.     

A numerical evaluation of combustion in porous media by EGM (Entropy Generation Minimization)

Combustion in Porous Media provides interesting advantages compared with the free flame combustion due to the higher burning rates, increased power dynamic range, the extension of lean flammability limits, and the low emissions of pollutants. A numerical code is developed in order to evaluate the effects of different parameters of combustion in porous media. The governing equations including Navier–Stokes, the solid and gas energy and the chemical species transport equations are solved using a multi-step reduced kinetic mechanism. Flame stabilization and the burner optimization are studied by EGM (Entropy Generation Minimization) method considering the effects of chemical affinities and reaction. It is found that the flames occurring at the upstream half of the porous layer are more stable and more efficient, producing less emissions than those occur at the downstream half of porous layer. Also at a specified equivalence ratio both the heat recirculation efficiency and the Merit number have similar trend by changing the flame location. For a FFL (Fixed Flame Location), there is an optimum value of equivalence ratio at which the burner efficiency is a maximum.     

多孔介质内往复流动下超绝热燃烧的实验研究

对RSCP的燃烧特性进行了实验研究．建成了RSCP实验台，它由泡沫陶瓷燃烧器、电磁阀控制的周期换向进排气管路系统和测量系统组成．对各种工况参数(燃料空气当量比、气体流速、循环半周期)下多孔介质内轴向温度分布进行了系统的测量．实验结果表明，较之常规的自由火焰燃烧器，RSCP具有增强火焰稳定性、拓宽燃料可燃极限等优点．对丙烷-丁烷混合气，其贫可燃极限可扩展到当量比0．065．在实验基础上，探讨了RSCP实现超绝热燃烧的机理，总结出有关工况参数对其燃烧特性影响的规律．     

Simulation of thermally‐enhanced combustion in a porous medium burner

Premixed combustion in a porous medium burner is investigated numerically. A two‐dimensional steady, laminar flow model is used. A single‐step reaction of methane is used for the chemical kinetic model. The model also includes thermal radiation transport of the porous media that is placed inside the burner. The radiative transport equation is solved by using the discrete ordinate method. The results show that, for each equivalence ratio, the flame can be stabilized at various axial locations with different flame speeds. The flame temperature increases with the equivalence ratio and flame speed. Furthermore, the energy release rates are much higher than that of a free flame for the same equivalence ratio as a result of higher flame speed. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(1): 75–88, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20088     

Experimental investigation of the performance of a liquid fuel-fired porous burner operating on kerosene-vegetable cooking oil (VCO) blends for micro-cogeneration of thermoelectric power

Studies related to porous burner for thermoelectric (TE) power generation have mainly focused toward achieving a specific range of power output for various applications. However, detailed analyses on the performance and emission aspects of the porous burner are lacking. In addition, physical integration between the burner and TE modules has added further complexity in this research area. Thus, this work aims to comprehend the effects of fuel–air equivalence ratio on the performance and emission characteristics of a liquid fuel-fired porous burner for micro-cogeneration of TE power. A catalytically inert Al₂O₃ porous medium was incorporated into a liquid fuel-fired porous burner operating on four mixtures of kerosene-vegetable cooking oil (VCO) blends: 100 kerosene, 90/10 KVCO, 75/25 KVCO, and 50/50 KVCO. Ten bismuth-telluride TE cells were arranged in a ten-sided polygon that, together with finned dissipators, formed a TE module electrically connected in series but thermally connected in parallel. The performance aspects at various fuel–air equivalence ratios were thoroughly evaluated with the corresponding temperature profiles, voltage, current, power output, and electrical efficiency. Results indicated that the surface temperature of the porous media was generally higher than the developed and exit flame temperature of the burner. Varying the fuel-air equivalence ratio significantly affected the electrical efficiency, with a maximum and minimum value of 1.94% and 1.10%, respectively. The power output steadily increased in the lean region, but stabilized as the fuel–air equivalence ratio slowly increased beyond the stoichiometric ratio. The CO emission was relatively lower at the lean region; however, significant amount was recorded in the rich combustion region. Moreover, NOx fluctuated between 1 ppm and 4 ppm over the entire range of fuel–air equivalence ratio.     

A Numerical and Experimental Investigation of Performance of a Nonsprayed Porous Burner

The performance of a nonsprayed porous burner (NSPB) is investigated through both numerical and experimental studies. The major requirement of liquid fuel combustion systems is excellent fuel vaporization, which is accomplished by using porous medium. Instead of heterogeneous combustion, which occurs in free space of a conventional sprayed burner, a homogeneous combustion of vaporized kerosene and air takes place within a porous medium. The liquid kerosene is preheated and completely vaporized in the first porous medium before being mixed with preheated air in the mixing chamber (i.e., a small space between two porous media). Then the combustion occurs in the second porous medium. A subcooled boiling, single global reaction combustion, and local nonthermal equilibrium between fluid and solid phases with phase change under complex radiative heat transfer are considered. The model accuracy is validated by the experimental data before parametric study—that is, equivalence ratio and firing rate are performed. Result show that a self-sustaining evaporation without atomization and matrix-stabilized flame can be achieved in the NSPB by providing the radiant output efficiency in the same range as a conventional premixed gaseous porous burner. This indicates that the NSPB is one possible technology to replace conventional spray burners for future requirements.     

多孔燃烧器中氨/空气燃烧特性数值研究

氨具有氢密度高、生产成本低、基础设施完善等优点,作为一种潜在的可再生替代燃料受到了广泛的关注.目前,仅有少数研究关注氨气燃烧喷嘴的研究,针对氨气稳定燃烧喷嘴的研究尤其不足.为实现氨燃料的稳定燃烧和低污染物排放,本研究提出了一种氨用多孔介质燃烧器.对氨用多孔介质燃烧器建立了二维数值模型,并对预混氨/空气在多孔介质燃烧器中...     

Porous burners for lean-burn applications

We review research on lean methane combustion in porous burners, with an emphasis on practical aspects of burner design and operation and the application of the technology to real-world problems. In particular we focus on ‘ultra-lean’ combustion, where the methane concentration is actually at or below the lean flammability limit for a free flame (5% methane by volume in air). Porous burners are an advanced combustion technology whereby a premixed fuel/air mixture burns within the cavities of a solid porous matrix. They are capable of burning low-calorific value fuels and very lean fuel/air mixtures that would not normally be flammable, potentially allowing the exploitation of what would otherwise be wasted energy resources. Possible lean-burn applications include the reburn of exhaust gases from existing combustion systems, and the mitigation of fugitive methane emissions. Porous burners operate on the principle that the solid porous matrix serves as a means of recirculating heat from the hot combustion products to the incoming reactants. This results in burning velocities higher than those for a free flame, as well as extended lean flammability limits. Burner performance is also characterised by low emissions of combustion related pollutants and stable operation over a wide range of fuel concentrations and flow rates. Stable combustion of methane/air mixtures below the conventional lean limit has been observed by a number of researchers; in one study the combustion of a mixture with a fuel concentration of only 1% was reported. A number of design considerations are important as regards optimising burner performance for lean-burn applications. Foremost among these is the selection of a suitable material for the porous matrix. Possibilities include packed beds of alumina spheres or saddles, and reticulated foams made of silicon carbide or high temperature metal alloys. Other potentially significant design issues include the length of the porous bed, the use of ‘multi-section’ designs where different porous materials are used in each section, the incorporation of external heat exchangers to supplement the heat recirculation provided by the porous matrix, and the ability to operate the burner at elevated pressures. There is an extensive body of research relating to porous burners, comprising experimental and numerical investigations. However the majority of previous studies have been directed towards the use of porous burners for radiant heating applications rather than for the combustion of low-calorific value fuels. Consequently there is a lack of reliable data relating specifically to ultra-lean combustion. We identify specific areas where further research is required to progress this field. These include the influence on burner performance of the design considerations listed above, the stability of the combustion process to fluctuations in fuel concentration and flow rate, the development of reliable models specifically for ultra-lean combustion in practical burners, and the investigation of issues relating to scale-up and commercial application.     

Radiation effects on flame stabilization on flat flame burners

This study investigates the impact of radiative heat transfer on the behavior of flat flame burners within the framework of a simplified one-dimensional model. Flat flame burners stabilize planar premixed flames downstream of a porous plug. Within this study, the porous plug is modeled as a thermally conducting, optically thick medium, allowing for both conductive and radiative heat transfer. Based on the simplified model, the impact of radiative heat exchange between the porous plug exit and the downstream environment is investigated. In “surface” combustion, flame stabilization occurs due to heat transfer between gas phase and porous solid. Results demonstrate that radiative heat transfer from a hot downstream environment to the porous plug significantly increases maximum attainable mass fluxes. For a cold downstream environment, plug properties do not affect the maximum supportable mass flux, although plug porosity and heat transfer between gas and solid have a significant impact on the “stand-off” distance between flame and plug exit. In addition, the model provides insight to a second “submerged” combustion mode, where the flame is stabilized within the porous plug of the burner. Here, increased flame temperatures lead to a dramatic increase of the maximum supportable mass flux. Overall, results show that radiative heat losses play a critical role in both combustion modes: in surface combustion, they are an important mode of heat dissipation, where they can prevent “flash-back” conditions with the flame moving into the porous matrix; in submerged combustion, they prevent flame stabilization close to inlet and exit faces and enable a “slow” solution branch that does not exist without radiative losses.     

Analytical and experimental study of premixed methane–air flame propagation in narrow channels

This study investigates analytically and experimentally the influence of preheat temperature on flame propagation and extinction of premixed methane–air flame in single quartz tubes with inner tube diameters of 3.9, 3, 2 and 1 mm respectively. The effects of preheat temperature, tube diameter, equivalence ratio and mixture flow rate on the flame speed and extinction conditions are determined. The analytical results show that high preheat temperature of the mixture can effectively suppress flame quenching, and the occurrence of stable solution in the slow flame branch extends the flammability limit leading to possible flame propagation in mini channels. Experimental results confirm that the flame speed increases and the flammability limit shifts toward the fuel lean direction either through increasing the preheat temperature or decreasing the mixture flow rate, or both. Decrease of propagating flame speed is observed before the stoichiometric equivalence ratio at high preheat temperatures. The analytical model provides insights into how propagating flame in mini channels can be sustained; however, the model is only good at predicting flame speed near the fuel lean branch. Influence of Cu²⁺ ions exchanged zeolite 13X catalyst on flame speed is also addressed. It is noted that the zeolite based catalyst can lower the preheat temperature requirement in order to sustain the flame propagation in narrow channels.     

Numerical simulation and theoretical analysis of premixed low-velocity filtration combustion

This paper investigates combustion wave characteristics of lean premixtures in a porous medium burner. Heat recuperation originated by the porous medium is examined by an one-dimensional numerical model. Attention is focused on the influences of solid properties, heat loss, equivalence ratio, etc., on the combustion wave speed and the maximum combustion temperature attained in the wave. Based on the flame sheet assumption a relationship between the combustion wave speed and the maximum combustion temperature is given. Then an approach from the laminar premixed flame theory is applied and the entire flame zone is divided into a pre-heating region and a reaction region, and treated separately. In this way, the second relationship between the two parameters is deduced. Thus a closed analytical solution for the combustion wave speed and the maximum combustion temperature is obtained. Over a wide range of working conditions, the numerical predictions and theoretical results show qualitative agreements with experimental data available from the literature. The results reveal that the mechanism of superadiabatic combustion is attributed to the overlapping of the thermal wave and combustion wave under certain conditions.     

多入口多孔介质燃烧器的数值模拟

在多入口燃烧器内加入多孔介质,以甲烷/空气为燃料,采用非预混燃烧的数值模拟方法,探究多入口燃烧器的燃烧情况.对比多孔介质燃烧与空间自由燃烧,分析了"超焓燃烧"现象;在多孔介质燃烧基础上,探究不同当量比对燃烧温度的影响;在多孔介质燃烧和不同当量比的基础上探究污染物CO和CO_2的排放情况.结果表明:多孔介质燃烧可以实现"超焓燃烧"特性,燃烧火焰温度高于自由空间燃烧温度;当量比对燃烧温度影响很大,随着当量比的增大,燃烧器内最高燃烧温度升高,但燃烧过程存在一个最佳当量比0.6,超过该当量比后最高温度将不再变化;多入口多孔介质燃烧有助于减少CO和CO_2的生成量.     

Experimental investigation of stabilization and emission characteristics of ammonia/air premixed flames in a swirl combustor

Ammonia is a possible candidate for use as a hydrogen energy carrier as well as a carbon-free fuel. In this study, flame stability and emission characteristics of swirl stabilized ammonia/air premixed flames were experimentally investigated. Results showed that ammonia/air premixed flame could be stabilized for various equivalence ratios and inlet flow velocity conditions in a swirl burner without any additives to enhance the reaction of ammonia even though the laminar burning velocity of ammonia is very slow. The lean and rich blowoff limits were found to be close to the flammability limits of the ammonia flame. In addition, emission characteristics were investigated using an FTIR gas analyzer. The NO concentration decreased and ammonia concentration increased under rich conditions. Moreover, it was found that there is an equivalence ratio in rich condition in which NO and ammonia emission are in the same order.     

Experimental investigation of premixed combustion limits of hydrogen and methane additives in ammonia

In this study, a specially designed premixed combustion chamber system for ammonia-hydrogen and methane-air laminar premixed flames is introduced and the combustion limits of ammonia-hydrogen and methane-air flames are explored. The measurements obtained the blow-out limits (mixed methane: 400–700 mL/min, mixed hydrogen: 200–700 mL/min), mixing gas lean limit characteristics (mixed methane: 0–82%, mixed hydrogen: 0–37%) and lean/rich combustion characteristics (mixed methane: ? = 0.6–1.9, mixed hydrogen: ? = 0.9–3.2) of the flames. The results show that the ammonia-hydrogen-air flame has a smaller lower blow-out limit, mixing gas ratio, lean combustion limit and higher rich combustion limit, thereby proving the advantages of hydrogen as an effective additive in the combustion performance of ammonia fuel. In addition, the experiments show that increasing the initial temperature of the premixed gas can expand the lean/rich combustion limits of both the ammonia-hydrogen and ammonia-methane flames.     

多孔介质稳焰器对旋流火焰振荡燃烧特性影响的试验研究

为研究多孔介质稳焰器孔密度变化对贫预混旋流火焰振荡燃烧特性的影响,通过光电倍增管测量全局火焰热释放率,采用双麦克风方法测量旋流器入口速度脉动,获得不同孔密度多孔介质稳焰器火焰传递函数;并通过高速相机测量不同孔密度多孔介质稳焰器振荡火焰结构的变化。试验结果表明：多孔介质能够改变燃烧室声模态,有效抑制燃烧振荡,但孔密度对受迫燃烧火焰热释放率和压力脉动影响具有非线性;高频入口扰动对火焰响应特性影响较弱,火焰受迫响应呈现低通滤波特性;火焰传递函数增益峰值对应入口激励频率存在差异,但相位分布斜率基本一致;多孔介质导致火焰向稳焰器中心轴线聚拢,相干结构更加明显;宽频扰动范围内的火焰张角分布趋势与火焰传递函数增益曲线的分布趋势相反。     

Understanding the influence of burner structure on the stability and chemiluminescence of inverse diffusion flame

Inverse diffusion lift-off flame was widely used in industrial fields such as non-catalytic partial oxidation of methane. In order to investigate the stability and chemiluminescence characteristics of the inverse diffusion lift-off flame, the OH1 and CH1 radiation characteristics, lift-off height, and transition (attachment, lift-off and blow-out) of flames under different burner structure were discussed. The results showed that burner rim thickness and incidence angles would affect the stability of the inverse diffusion flame. When the thickness of the burner rim exceeded 0.5 mm, the flame would directly change from the attachment state to blow-out state as oxygen velocity increased. Additionally, the values of the blowout limit of the nozzle were inversely proportional to the rim thickness of the burner. Different incident angles would result in various shear angles, which would affect the flame structure. As the incidence angle decreased, the tangential velocity of flame increased and the flame tended to be more stable. When the lift-off flame generated, OH1 intensity and distribution showed a sudden change, and the OH1/CH1 peak intensity ratio of the flame appeared abrupt changes.     

Plasma-assisted stabilization of laminar premixed methane/air flames around the lean flammability limit

Studies of nanosecond repetitively pulsed discharge plasma-assisted combustion were carried out on a laminar premixed methane/air flow in the vicinity of the lean flammability limit. Experimental results indicated that complete combustion is achieved when the equivalence ratio is above the known lean flammability limit (? = 0.53) at high discharge repetition rates of 50 kHz. When the ratio is below the limit, the plasma does serve as a flame holder; however, only partial combustion is seen in the downstream flow. Two-dimensional kinetic simulation results were found to be consistent with the experimental results. The simulations revealed that the methane is fully consumed within the discharge region, since the time between discharge pulses is less than that for species diffusion and advection. This creates a source of radicals and high temperature, which diffuse outwards to stabilize the combustion of the surrounding flow.