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.     

Numerical and experimental investigation of a mild combustion burner

An industrial burner operating in the MILD combustion regime through internal recirculation of exhaust gases has been characterized numerically. To develop a self-sufficient numerical model of the burner, two subroutines are coupled to the CFD solver to model the air preheater section and heat losses from the burner through radiation. The resulting model is validated against experimental data on species concentration and temperature. A 3-dimensional CFD model of the burner is compared to an axisymmetric model, which allows considerable computational saving, but neglects some important burner features such as the presence of recirculation windows. Errors associated with the axisymmetric model are evaluated and discussed, as well as possible simplified procedures for engineering purposes. Modifications of the burner geometry are investigated numerically and suggested in order to enhance its performances. Such modifications are aimed at improving exhaust gases recirculation which is driven by the inlet air jet momentum. The burner is found to produce only 30 ppm_v of NO when operating in MILD combustion mode. For the same air preheating the NO emissions would be of approximately 1000 ppm_v in flame combustion mode. It is also shown that the burner ensures more homogeneous temperature distribution in the outer surfaces with respect to flame operation, and this is attractive for burners used in furnaces devoted to materials' thermal treatment processes. The effect of air excess on the combustion regime is also discussed.     

Mechanisms of stabilization and blowoff of a premixed flame downstream of a heat-conducting perforated plate

The objective of this work is to investigate the flame stabilization mechanism and the conditions leading to the blowoff of a laminar premixed flame anchored downstream of a heat-conducting perforated-plate/multi-hole burner, with overall nearly adiabatic conditions. We use unsteady, fully resolved, two-dimensional simulations with detailed chemical kinetics and species transport for methane-air combustion. Results show a bell-shaped flame stabilizing above the burner plate hole, with a U-shaped section anchored between neighboring holes. The base of the positively curved U-shaped section of the flame is positioned near the stagnation point, at a location where the flame displacement speed is equal to the flow speed. This location is determined by the combined effect of heat loss and flame stretch on the flame displacement speed. As the mass flow rate of the reactants is increased, the flame displacement speed at this location varies non-monotonically. As the inlet velocity is increased, the recirculation zone grows slowly, the flame moves downstream, and the heat loss to the burner decreases, strengthening the flame and increasing its displacement speed. As the inlet velocity is raised, the stagnation point moves downstream, and the flame length grows to accommodate the reactants mass flow. Concomitantly, the radius of curvature of the flame base decreases until it reaches an almost constant value, comparable to the flame thickness. While the heat loss decreases, the higher flame curvature dominates thereby reducing the displacement speed of the flame base. For a stable flame, the gradient of the flame base displacement speed normal to the flame is higher than the gradient of the flow speed along the same direction, leading to dynamic stability. As inlet velocity is raised further, the former decreases while the latter increases until the stability condition is violated, leading to blowoff. The flame speed during blow off is determined by the feedback between the growing recirculation zone and the cooling burner plate.     

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.     

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.     

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.     

Improvement of hollow cylinders on the conversion of coal mine methane to hydrogen in packed bed burner

Coal mine methane (CMM) combustion in the porous media burner (PMB) of boiler system contributes to the simultaneous production of hydrogen and heat, and the hydrogen can be used as the main raw materials for solid oxide fuel cell (SOFC). In this study, the double-layer burners were built by filling the downstream with the hollow cylinders of different sizes and pore numbers due to the high porosity of hollow-structure units compared with the common packed bed of pellets. The distributions of temperature, species concentration and reforming efficiency were obtained on the consideration of operating condition and preheating temperature. Results shows that the reforming efficiency of the 10-6-10 mm cylinder burner was optimal in all one-pore cylinder burners with the highest concentration of 12.3% (H₂) and 8.8% (CO). As for the packed bed of 8 mm cylinders, the four-pore cylinder burner showed the highest peak temperature and maximum yields of hydrogen. With the increasing of preheating temperature, the stabilization time of the flame propagating decreased. Moreover, the peak temperature and reforming efficiency increased with the increasing of the inlet velocity and the largest efficiency of 54.2% appeared at the velocity of 18 cm/s in the 10 mm cylinder burner.     

Mechanism analysis on the pulverized coal combustion flame stability and NOx emission in a swirl burner with deep air staging

Low NOx burner and air staged combustion are widely applied to control NOx emission in coal-fired power plants. The gas-solid two-phase flow, pulverized coal combustion and NOx emission characteristics of a single low NOx swirl burner in an existing coal-fired boiler was numerically simulated to analyze the mechanisms of flame stability and in-flame NOx reduction. And the detailed NOx formation and reduction model under fuel rich conditions was employed to optimize NOx emissions for the low NOx burner with air staged combustion of different burner stoichiometric ratios. The results show that the specially-designed swirl burner structures including the pulverized coal concentrator, flame stabilizing ring and baffle plate create an ignition region of high gas temperature, proper oxygen concentration and high pulverized coal concentration near the annular recirculation zone at the burner outlet for flame stability. At the same time, the annular recirculation zone is generated between the primary and secondary air jets to promote the rapid ignition and combustion of pulverized coal particles to consume oxygen, and then a reducing region is formed as fuel-rich environment to contribute to in-flame NO_X reduction. Moreover, the NOx concentration at the outlet of the combustion chamber is greatly reduced when the deep air staged combustion with the burner stoichiometric ratio of 0.75 is adopted, and the CO concentration at the outlet of the combustion chamber can be maintained simultaneously at a low level through the over-fired air injection of high velocity to enhance the mixing of the fresh air with the flue gas, which can provide the optimal solution for lower NOx emission in the existing coal-fired boilers.     

The infrared thermograph observation of a porous medium assisted catalyst packed-bed under excess enthalpy reforming

This paper investigated the design of a porous medium–catalyst hybrid reformer for CO₂ conversion by dry auto-thermal reforming, with emphasis on the reaction temperature distribution. In the reforming process, the reaction under excess enthalpy was explored by interface temperature measurement and infrared thermograph observation in a porous medium assisted packed-bed catalyst reactor. The hybrid design was arranged with the catalytic packed-bed in the downstream of the porous medium. In the arrangement, the reactants were preheated by internal heat recirculation and the conversion of CO₂ was enhanced by the catalyst surface reaction. The temperature measurement at the axial position and infrared thermograph observation on the catalyst packed-bed indicated that the peak temperature could be stabilized and held at the interface of the PM and catalyst bed, which significantly improved the propagation and stability of the flame. This was helpful to the improvement of both thermal wave transfer and internal heat recirculation.     

Numerical studies on the combustion of ultra-low calorific gas in a divergent porous burner with heat recovery

This study presents the idea of heat recovery through recirculating walls to enhance the combustion stability for ultra-low calorific gas in a porous burner. Numerical studies on the combustion of ultra-low calorific gas of CO/H₂ with CO₂ and N₂ in a developed divergent porous burner with annular channel is conducted using two-dimensional axis symmetrical model with detailed kinetics. The heat recovery efficiency is defined as the ratio of heat recovery by the fresh mixture in the annular channel to burner power. It is shown that the heat recovery has significant effect on the minimal inlet gas temperature (MIGT) for stable combustion. It is confirmed that the heat recovery enhances the combustion and the stability limits are enlarged by preheating the fresh mixture, but it also leads to an extra pressure loss across the burner compared to that without heat recovery. Results show that heat recovery efficiency reaches up to 0.18 for all the investigated parameters and it reduces linearly from 0.32 to 0.18 as the mass flow ratio increases from 0.8 to 1.5. The MIGT for the burner with heat recirculating channel is always smaller than that without heat recovery. As a result, the combustion is greatly improved by the heat recovery in the divergent burner. Meanwhile, it is shown that pressure loss is increased significantly when the heat recirculating annular channel is added.     

Burning rates and temperatures of flames in excess-enthalpy burners: A numerical study of flame propagation in small heat-recirculating tubes

This study investigates flame propagation in small thermally-participating tubes where the wall acts as a heat-recirculating medium. This fundamental configuration allows heat in the combustion products to be recirculated into the reactants, resulting in excess enthalpy and enhanced burning rates. Preheating of the reactants by heat recirculation has traditionally been considered to be the dominant mechanism leading to large burning rates observed in such systems. This is mainly supported by results from physical models based on a one-dimensional (1-D) representation of the system, where the radial diffusion of heat from wall surface to channel centerline is not accurately captured. In this study, a 2-D formulation with conjugate heat transfer, which accurately resolves the transport of heat inside the gas-wall system, is used to model the excess-enthalpy phenomenon. Steadily-propagating stoichiometric methane–air flames are simulated inside an adiabatic tube of finite wall-thickness, over a wide range of inlet flow velocities and small tube diameters. Burning-rate enhancement is found to be caused not only by preheating, associated with heat recirculation, but also by an increase in flame-front area. Flame elongation is more pronounced with increasing tube diameter and inlet velocity, up to a point where the change in flame-front area becomes dominant in enhancing burning rate. In that case, heat recirculation is a necessary condition for flames to couple to the thermal wave in the wall and elongate, but does not provide a significant increase in enthalpy or temperature that would otherwise be needed for high burning rates to be observed. As the diameter is reduced, the effect of preheating becomes increasingly important for burning-rate enhancement compared to flame area increase. At very small diameters, smaller than the flame thickness, the increase in burning rate is seen to be predominantly attributable to preheating. However, preheating is seen to become limited as inflow velocity is increased, due to 2-D effects inside the fluid that interfere with heat recirculation. These findings demonstrate that 2-D effects inside the fluid can have a prohibitive influence on the burning-rate enhancement attributed to preheating, but that they also give rise to an additional mechanism, associated with the change in flame surface area, responsible for burning-rate enhancement in heat-recirculating burners.     

Combustion of suspended fine solid fuel in air inside inert porous medium: A heat transfer analysis

Present work is a numerical analysis of combustion of submicron carbon particles inside an inert porous medium where the particles in form of suspension in air enter the porous medium. A one-dimensional heat transfer model has been developed using the two-flux gray radiation approximation for radiative heat flux equations. The effects of absorption coefficient, emissivity of medium, flame position and reaction enthalpy flux on radiative energy output efficiency have been presented. It is revealed that in porous medium the combustion of suspended carbon particles is similar to premixed single phase gaseous fuel combustion except the former has shorter preheating temperature zone length. Use of porous ceramic having high porosity and made of Al₂O₃ or ZrO₂ with stabilized flame position operated nearer to downstream end will ensure radiative output maximum and minimum at downstream and upstream end, respectively.     

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...     

甲烷/高温烟气稀释空气的Mild燃烧过程的数值研究

中低氧浓度稀释MILD燃烧与传统有焰燃烧方式相比,燃烧室内温度相对均匀,可以有效控制和降低NO_X等污染物排放。燃烧室内的烟气循环导致的混合气稀释程度、预热温度以及流场应变率对Mild燃烧过程有重要影响。通过对不同稀释和预热程度的一维层流对撞扩散火焰进行计算,研究了烟气回流稀释/预热形成的Mild燃烧情况。数值计算结果表明,Mild燃烧能很好的控制燃烧场的最高温度和反应放热速率,得到较为均匀的温度场,并因此降低产物中污染物的浓度。对不同预热温度、不同拉伸率下工况的计算证明了Mild燃烧能够扩展燃料的燃烧极限,Mild燃烧对流场等外部干扰的敏感度较小,燃烧稳定。     

A combustion concept for oxyfuel processes with low recirculation rate – Experimental validation

Oxyfuel combustion is a technology for Carbon Capture & Storage from coal fired power plants. One drawback is the large necessary amount of recirculation of cold flue gases into the combustion chamber to avoid inadmissible high flame temperatures. The new concept of Controlled Staging with Non-stoichiometric Burners (CSNB) makes a reduction of the recirculation rate possible without inadmissible high flame temperatures. This reduction promises more compact boiler designs. We present in this paper experiments with the new combustion concept in a 3 × 70 kW natural gas combustion test rig with dry flue gas recirculation of 50% of the cold flue gases. The new concept was compared to a reference air combustion case and a reference oxyfuel combustion case with recirculation of 70% of the cold flue gases. FTIR emission spectroscopy measurements allowed the estimation of spectral radiative heat fluxes in the 2–5.5 μm range. The mixing of the gases in the furnace was good as the burnout and the emissions were comparable to the reference cases. The flame temperatures of the CSNB case could be controlled by the burner operation stoichiometry and were also similar to the reference cases. The heat flux in the furnace through radiation to the wall was higher compared to the oxyfuel reference case. This is an effect of the lowered recirculation rate as the mass flow out of the furnace and therefore the sensible heat leaving the furnace decreases. The higher oxygen consumption with lower recirculation rate could be compensated by a lower furnace stoichiometry. This was possible due to better burnout with increased oxygen concentrations in the burner. The results prove that a reduction of the flue gas recirculation rate in oxyfuel natural gas combustion from 70% down to 50% is possible while avoiding inadmissible high flame temperatures with the concept of Controlled Staging with Non-stoichiometric Burners.     

The effects of hydrogen addition,inlet temperature and wall thermal conductivity on the flame-wall thermal coupling of premixed propane/air mixtures in meso-scale tubes

The effects of hydrogen addition, inlet temperature, wall thermal conductivity and wall thickness on the flame-wall coupling of the propane/air flames in a meso-scale tube are numerically investigated using a two dimensional model along with the detailed chemical mechanism. Higher wall thermal conductivity can result in preheating the fresh mixture uniformly in strongly flame-wall coupled system, which is vital to enhance the burning rate of fuel mixture. With the increase of wall thermal conductivity or hydrogen addition, the leading edge of the flame shifts from the wall to the axis, meanwhile the flame is more convex towards the unburned side near the leading edge. As the hydrogen addition and inlet temperature increase, the flame propagation speed increases significantly, while the maximum temperature and maximum total enthalpy decrease due to the reduced heat recirculation power. The flame propagation speed has a negative correlation with heat loss. The chemical reactions in preheat zone are enhanced at low wall thermal conductivity due to the higher inner wall temperature. Thinner combustor wall leads to higher flame speed and higher heat loss simultaneously. Results have implications on the choice of solid wall material and heat recirculation design in a stable meso-scale combustor for different fuels.     

入口旋流数对煤粉低尘燃烧器流场的影响

以液排渣旋风燃烧过程为基础的煤粉低尘燃烧器可在燃烧过程实现捕渣,为工业加热提供低含尘浓度的高温火焰,是工业加热过程实现以煤代油的先进燃烧技术。根据旋流燃烧流动特点,采用能考虑非均向湍流应力的雷诺应力模型,对旋流煤粉低尘燃烧器内气流流动过程场进行数值模拟计算,计算结果与流场实验测试相吻合。研究表明,气流进入燃烧器时的旋转强度（旋流数）对燃烧器内的流动特性有很大影响,在冷态模型条件下,当旋流数在7以上时,环室回流在轴向贯穿燃烧器整个流场,有利于增加煤粉颗粒在燃烧室内的循环次数,提高灰渣捕获率;低于7时,环室回流出现阻断,不再连续,易造成煤粉颗粒直接逸出,对燃烧及灰渣捕获不利。随旋流数增加,燃烧器出口处中心回流率增大,对炉膛高温烟气的抽吸作用增强。     

转底炉内温度场及流场的数值模拟

利用Fluent软件建立了转底炉炉内温度场及流场的数学模型,采用κ-ε模型,辐射模型,非预混燃烧模型模拟炉内的辐射传热和湍流流动,分析了烧嘴和助燃风喷嘴的选择与布置对炉内的温度及速度分布的影响.结果表明,转底炉预热段与加热段采用炉顶布置平焰烧嘴供燃料,炉膛下部布置喷嘴供助燃风能获得很好的温度场和流场来满足工作需要;还原段选择蓄热式烧嘴供给燃料,即满足了高温还原的温度要求,也达到充分回收利用烟气余热的效果.     

A filtered-laminar-flame PDF sub-grid-scale closure for LES of premixed turbulent flames: II. Application to a stratified bluff-body burner

Large eddy simulation of the two stratified nonswirling configurations of the Cambridge burner studied by Sweeney et al. (2012) is presented. The sub-grid-scale combustion closure relies on a physical space filtering operation with a filter size determined locally depending on the resolved and sub-grid-scale flame properties, which is discussed in a companion paper. Similarly to the premixed configuration of the same burner, the modeling reproduces the differential diffusion effects leading to accumulation of carbon and an enhancement of mixture fraction in the recirculation zone, an effect that is less pronounced than in the fully lean premixed case, because of the modification of the topology of the reaction zone that is induced by the mixture stratification. The study of the LES combustion regimes shows that the reaction zones develop under a quite large range of flame topologies, from wrinkled flamelets up to thin reaction zones. Instantaneous and time-averaged LES data were analyzed to extract information concerning the degree of stratification and the orientation of flame and mixing vectors. A decomposition of the flame response into premixed, diffusion, and partially premixed flamelets is performed, to conclude that the premixed mode dominates close to the burner, with a partially premixed burning regime further downstream. Overall, the length scales associated with stratification were found to be much larger than that of the reaction zone and flame, resulting in a quasi-homogeneous propagation, predominantly in a back supported stratified combustion regime. Overall good agreement between simulation and measurements was obtained for either configurations.