Small scale porous medium combustion system for heat production in households

For heating purposes in modern households, gas burners are normally applied due to their simplicity, low cost and easy handling. On the other hand, practical experience showed that conventional, open flame gas burners compared to porous medium systems have low dynamic range, i.e. low power modulation capability and, additionally, higher production of pollutants such as carbon monoxide (CO) and nitrogen oxides (NO_x). This is especially notable when the burner operates at low thermal power regimes. In order to avoid the above-mentioned difficulties and disadvantages of conventional burners, new porous medium gas burner system with maximum thermal output of 8 kW has been developed. The objective of the presented work is focussed on better understanding and enabling further developing of porous medium burners (PMB) for household heating systems. The aim of the work is also to develop a compact and highly efficient combined heating system based on the 8 kW gas PM burner coupled with a new heat exchanger incorporated into an electro-fossil fuel system considering space and domestic water heating in one-family house. The final result was the heating system with modulation of the thermal power up to approximately 1:8 and low emissions of CO and NO_x.     

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.     

Development and characterization of a low-NOx partially premixed hydrogen burner using numerical simulation and flame diagnostics

The utilization of hydrogen as a fuel in free jet burners faces particular challenges due to its special combustion properties. The high laminar and turbulent flame velocities may lead to issues in flame stability and operational safety in premixed and partially premixed burners. Additionally, a high adiabatic combustion temperature favors the formation of thermal nitric oxides (NO). This study presents the development and optimization of a partially premixed hydrogen burner with low emissions of nitric oxides. The single-nozzle burner features a very short premixing duct and a simple geometric design. In a first development step, the design of the burner is optimized by numerical investigation (Star CCM+) of mixture formation, which is improved by geometric changes of the nozzle. The impact of geometric optimization and of humidification of the combustion air on NO_x emissions is then investigated experimentally. The hydrogen flame is detected with an infrared camera to evaluate the flame stability for different burner configurations. The improved mixture formation by geometric optimization avoids temperature peaks and leads to a noticeable reduction in NO_x emissions for equivalence ratios below 0.85. The experimental investigations also show that NO_x emissions decrease with increasing relative humidity of combustion air. This single-nozzle forms the basis for multi-nozzle burners, where the desired output power can flexibly be adjusted by the number of single nozzles.     

Combustion characteristics of mixture of anode off gas and LNG in reformer

Fuel processing system which converts hydrocarbon fuel into hydrogen rich gas (by stream reforming, partial oxidation, auto-thermal reforming) needs high temperature environment (600-1000 °C). Generally, anode off gas or mixture of anode off gas and LNG are used as input gas for a fuel reformer. In order to constitute efficient and low emission burner system for fuel reformer, it is necessary to elucidate the combustion and emission characteristics of fuel reformer burner. In this study, lean flat flame using the ceramic porous burner was analyzed numerically and experimentally. Burning velocity of anode off gas calculated by CHEMKIN simulation was 51.8 cm, which was faster than that of LNG having 40.63 cm/s at the stoichiometric ratio because of high composition of hydrogen in anode off gas. As composition of LNG in mixture of anode off gas + LNG is increased, the burning velocity decreases and in the other hand the adiabatic temperature increases. CO, NO_x were measured below 50 ppm in operating load range of the reformer. Blue flame pattern was found as stable flame region for design of fuel reformer and anode off gas flame was maintained in blue flame pattern at equivalence ratio 0.55-0.62 under 1-5 kW power range.     

Investigation of NOx conversion characteristics in a porous medium

The conversion of nitric oxide (using CNG/air as fuel/oxidizer) inside a porous medium is investigated in this study. Unlike freely propagating flames, porous burners provide a solid medium that facilitates heat exchange with the gaseous phase. The heat exchange allows the stabilization of a variety of fuel mixtures from lean to rich and with a variety of calorific values. In addition, it allows the control of the reaction zone temperature and thus the control of pollutant formation while maintaining flame stability. An experimental porous burner was designed and manufactured for this purpose. The effects of equivalence ratio and flow velocity on the flame stabilization, NO_x and TFN (total fixed nitrogen) conversion ratios, and temperature profiles along the burner are investigated. In addition, numerical calculations using the PLUG flow simulator model and the GRI 3.0 kinetic mechanism reveals the key reactions which control the conversion efficiency. It was found that under slightly fuel-rich conditions (φ?1.3) NO_x mostly converts to N₂ with a maximum conversion ratio of 65%, while for higher equivalence ratios (φ>1.3) a large proportion of NO_x converts to NH₃. Results from experiments and numerical modeling showed that the temperature profile along the burner has significant effects on the NO_x and TFN conversion ratios. It was also found that temperatures between 1000 and 1500 K are most desirable for NO_x and TFN conversion in the porous burner. Analysis of the chemical paths for the low- and high-equivalence-ratio cases showed that the formation of nitrogen-containing species under very rich conditions (φ>1.3) is due to the increased importance of the HCNO path as compared to the HNO path. The latter is the dominant path at low equivalence ratios (φ?1.3) and leads to the formation of N₂. The NO concentration in the initial mixture was found to improve the conversion by up to 20% at low equivalence ratios (φ?1.3) and to have negligible effect at higher equivalence ratios.     

Design and development of a SPMB (self-aspirating, porous medium burner) with a submerged flame

This work reports design and development of a SPMB (self-aspirating porous medium burner) for replacing the self-aspirating, CB (conventional gaseous fuel, free flame burners), which are widely used in heating process of SMEs (small and medium scale enterprises) in Thailand but they have relatively low thermal efficiency of about 30 percent. Design of the SPMB relies on the same important characteristics of the CB, i.e. using the same mixing tube and the same fuel nozzle. The SPMB is formed by a packed bed of alumina spheres. The pressure drop across the packed bed, diameter of particles and a combustion chamber diameter are estimated by Ergun’s equation in combination with Pe (Peclet number). The SPMB yields a submerged flame with an intense thermal radiation emitted downstream. An output radiation efficiency as high as 23 percent can be achieved at relatively high turn-down ratio of 2.65 and firing rate ranging from 23 to 61 kW. The SPMB shows a more complete combustion with relatively low CO emission of less than 200 ppm and acceptably high NO_x emission of less than 98 ppm as compared with the CB throughout the range of firing rate studied, suggesting the possibility of the SPMB in replacing the CB.     

Development of energy efficient porous medium burners on surface and submerged combustion modes

Two compact premixed LPG burners based on submerged and surface combustion modes in porous medium (abbreviated as MSB and SSB respectively) are developed and their combustion and emission characteristics are compared to those of the CB (conventional burner). The preheating and reaction zones of MSB are made from porcelain form and Alumina spheres of 30 mm size, respectively, and the corresponding zones in SSB are made from Alumina (Al₂O₃) foams of pore densities 26 ppcm and 8 ppcm. NO_x emission is reduced by 76% and 75% by the use of MSB and SSB, respectively, compared with the CB, with acceptable CO and SO₂ emissions. For a thermal load of 0.62 kW, the thermal efficiencies of CB, MSB and SSB are estimated to be 47%, 59% and 71%, respectively.     

Modeling fuel NOx formation from combustion of biomass-derived producer gas in a large-scale burner

This study investigates the characteristics of fuel NO_x formation resulting from the combustion of producer gas derived from biomass gasification using different feedstocks. Common industrial burners are optimized for using natural gas or coal-derived syngas. With the increasing demand in using biomass for power generation, it is important to develop burners that can mitigate fuel NO_x emissions due to the combustion of ammonia, which is the major nitrogen-containing species in biomass-derived gas. In this study, the combustion process inside the burner is modeled using computational fluid dynamics (CFD) with detailed chemistry. A reduced mechanism (36 species and 198 reactions) is developed from GRI 3.0 in order to reduce the computation time. Combustion simulations are performed for producer gas arising from different feedstocks such as wood gas, wood + 13% DDGS (dried distiller grain soluble) gas and wood + 40% DDGS gas and also at different air equivalence ratios ranging from 1.2 to 2.5. The predicted NO_x emissions are compared with the experimental data and good levels of agreement are obtained. It is found out that NO_x is very sensitive to the ammonia content in the producer gas. Results show that although NO–NO₂ interchanges are the most prominent reactions involving NO, the major NO producing reactions are the oxidation of NH and N at slightly fuel rich conditions and high temperature. Further analysis of results is conducted to determine the conditions favorable for NO_x reduction. The results indicate that NO_x can be reduced by designing combustion conditions which have fuel rich zones in most of the regions. The results of this study can be used to design low NO_x burners for combustion of gas mixtures derived from gasification of biomass. One suggestion to reduce NO_x is to produce a diverging flame using a bluff body in the flame region such that NO generated upstream will pass through the fuel rich flame and be reduced.     

Combustion Wave Propagation of a Modular Porous Burner with Annular Heat Recirculation

A numerical investigation of the different arrangements of porous media in a combustor with annular heat recirculation is conducted.The effect of annular heat recirculation and porous block arrangement on the characteristics of combustion wave propagation is numerically studied.Results show that power input,heat capacity of porous matrix,arrangement of porous blocks,and annular heat recirculation are major factors that influence the propagation of combustion wave.The overall temperature of ceramic porous burner is higher than that of ceramic-metal type burner due to the lower heat storage capacity of the former,especially for the temperature downstream.The flame temperature is higher upstream and lower downstream with metal foams in the annulus than that without metal foams.The flame temperature of uniformity type burner is more uniform than that of gradually-varied and modular type burners.The flame front moves more slowly with metal foams in the annulus than that without metal foams due to the better preheating effect of metal foams.The flame position moves downstream,and the flame temperature gradually decreases and is eventually extinguished due to the low preheating temperature.     

Pilot impact on turbulent premixed methane/air and hydrogen-enriched methane/air flames in a laboratory-scale gas turbine model combustor

The impact of pilot flame operation on the combustion of pure methane and hydrogen-enriched methane (H₂/CH₄: 50/50 in vol%) fuels was investigated in a gas turbine model combustor under atmospheric conditions. The burner assembly was designed to mimic the geometry of an industrial burner, the Siemens DLE Burner, in which a concentric annular ring equipped with pilot flame burners is implemented in the dome of the combustor. Two pilot burner configurations have been investigated: a non-premixed and a partially premixed pilot arrangement. The performance of the pilot burners was evaluated for varying Reynolds number (Re) and H₂ enrichment. High-speed OH1 chemiluminescence imaging, as well as simultaneous planar laser-induced fluorescence measurements of the OH radicals and formaldehyde (CH₂O) were used for evaluating the dynamics and structures of the flames for different conditions. Furthermore, emission measurements were carried out to determine the influence of hydrogen dilution on the NO_x and CO emission levels. The main findings are (a) the effect of the pilot flame is sensitive to the Reynolds number of the main flame and the type of the pilot flame, (b) the stability range becomes narrower with increasing hydrogen ratio, due to the tendency to flashback, (c) non-premixed pilot flames lower the NO_x and increase the CO emissions, albeit rather small differences in the emissions have been detected, and (d) the NO_x and CO emissions become significantly lower with increasing hydrogen ratio.     

Fuel‐lean VOCs combustion in a porous burner stacked with alumina balls: A case for ethylene combustion

A porous burner stacked in turn with 3‐ and 9‐mm alumina pellets was established to perform C₂H₄ combustion experiments by acquiring the flammable limits, temperature variation characteristics, combustion wave velocity, pollutant emissions, and treatment efficiency. The burner operated well at equivalence ratios within 0.3 to 0.7. Larger alumina pellets widened the burner's lower flammable limit. As the flame propagated downstream, the higher premixed gas flow velocity and larger alumina pellets, the higher combustion wave velocity, whereas the circumstances were opposite as the flame spread upstream. The combustion temperature increased with the equivalence ratio and premixed gas flow velocity. In response to the effect of the alumina pellet dimension, 3‐mm alumina pellets corresponded to higher combustion temperatures, lower CO emissions, and higher treatment efficiency than those less than 9‐mm conditions.     

Combustion of hydrogen in a bubbling fluidized bed

The combustion of hydrogen in a hot, bubbling bed of quartz sand fluidized by air has been studied for the first time, by injecting hydrogen just above the distributor, via six horizontal fine tubes of Cr/Ni. Overall the fluidizing gas was oxygen-rich, with the composition varying from nearly stoichiometric to very lean mixtures. With the bed initially fluidized at room temperature, combustion (after ignition by a pilot flame) occurs in a premixed flame sitting on top of the bed. When the sand warms up, combustion becomes explosive in bubbles leaving the bed, exactly as with a hydrocarbon as fuel. However, in contrast to hydrocarbons, it is clear that when the bed reaches 500-600 °C, heat is produced both above the top of the bed (because of H₂ bypassing the bed) and very low down in the bed. In fact, with hydrogen as fuel, the location of where bubbles ignite descends abruptly to low in the sand; furthermore, the descent occurs at ∼500 °C, which is ∼100 K below the ignition temperature predicted by well-established kinetic models. However, the kinetic models do reproduce the observations, if it is assumed that the Cr/Ni hypodermic tubes, through which the fuel was injected, exert a catalytic effect, producing free H atoms, which then give rise to HO₂ radicals. In this situation, kinetic modeling indicates that bubbles ignite when they become sufficiently large and few enough to have a lifetime (i.e. the interval between their collisions) longer than the ignition delay for the temperature of the sand. The amounts of NO found in the off-gases were at a maximum (24 ppm), when the bed was at ∼500 °C for λ=[O₂]/stoich[O₂]=1.05. The variations of [NO] with [air]/[H₂] and also temperature indicate that NO is produced, at least partly, via the intermediate N₂H. In addition, the air-afterglow emission of green light (from NO+O→NO₂+hν) was observed in the freeboard, indicating the presence there of both NO and free atoms of oxygen for 1.05<λ<1.1.     

Characterization of confined hydrogen-air jet flame in a crossflow configuration using design of experiments

Hydrogen combustion has many industrial applications and development of new hydrogen burners is required to fulfil new demands. A novel configuration of hydrogen burner utilizing crossflow injection of fuel jets into swirling combustion air is characterized empirically in this work. It is intended as a first step in the development of new burner technologies having reduced emission levels and improved efficiency. Experiments were designed using the full factorial design method. Operating parameters were varied simultaneously and the NO_X emissions from the flame stabilized on the burner were measured. Statistical analysis of the experimental data showed that overall equivalence ratio is the dominant factor and lower NO_X emissions are observed at low equivalence ratios, irrespective of the burner power level. The analysis yielded an empirical relationship among NO_X emission, overall equivalence ratio, and power level that is useful in the design activity for a future combustion system based on the proposed configuration.     

Numerical investigation of premixed hydrogen/air combustion at lean to ultra-lean conditions and catalytic approach to enhance stability

Premixed combustion of hydrogen/air over a platinum (Pt) catalyst is numerically investigated in a planar channel burner with the aim of stabilising the flame at lean to ultra-lean conditions. A steady laminar species transport model is examined in conjunction with elementary heterogeneous and homogeneous chemical reaction schemes and validated against experimental results. A stability map is obtained in a non-catalytic burner for the equivalence ratios (φ) of 0.15–0.20, which serves as the basis for the catalytic flame analysis. Over the Reynolds numbers (Re) investigated in the non-catalytic burner, no flame is observed for φ ≤ 0.16, and flame extinction occurs at Re < 571 and Re < 381 for φ = 0.18 and 0.20, respectively. Moreover, a significant amount of unburned H₂ exits the burner in all cases. With the Pt catalyst coated on the walls, complete H₂ combustion is attained for 0.10 ≤ φ ≤ 0.20 where the contribution of gas phase (homogeneous) reaction increases with Re. Furthermore, radiation on the wall and at the inlet affects the combustion kinetics and flame temperature. Finally, NO_x emission is investigated under the same conditions and found to increase with equivalence ratio but has a negligible effect with the inflow Reynolds number.     

Combustion characteristics of a swirling inverse diffusion flame upon oxygen content variation

The combustion characteristics of a swirling inverse diffusion flame (IDF) upon variation of the oxygen content in the oxidizer were experimentally studied. The oxidizer jet was a mixture mainly composed of oxygen and nitrogen gases, with a volumetric oxygen fraction of 20%, 21% and 26%, and liquefied petroleum gas (LPG) was used as the fuel. Each set of experiment was conducted with constant oxygen content in the oxidizer. When the oxygen was varied, the changes in flame appearance, flame temperature, overall pollutant emission and heating behaviors of the swirling IDF were investigated. The swirling IDFs with different oxygen content in the oxidizer have similar flame structure involving a large-size and high-temperature internal recirculation zone (IRZ) which favors for thermal NO formation, and the thermal mechanism dominates the NO production for the swirling IDFs. The use of nitrogen-diluted air (with 20% oxygen) allowed the IDFs to operate at lower temperature with reduced NO_x formation, compared to the case of air/LPG combustion (with 21% oxygen). Meanwhile, an increase in CO emission is observed. With oxygen-enriched air (26% oxygen), the increase in temperature and EINO_x under lean conditions is more significant than under rich conditions. With 26% oxygen in the oxidizer stream, the IDF produces: (1) a shorter and narrowed navy-blue flame ring located closer to the burner exit, (2) highly luminous yellow flame extending into the central IRZ and above the blue flame ring, (3) a low CO emission, especially under lean conditions, (4) an increase in temperature at low Ф while a decrease in temperature at high Ф, and (5) an increase in EINO_x at all Ф. The heating test using the swirling IDFs in flame impingement heat transfer reveals that the heating rate can be monotonically increased as oxygen content in the oxidizer jet increases under the lean condition (Ф = 1.0). The oxygen enrichment does not contribute to the heating rate under the rich condition (Ф = 2.0), because for the non-premixed combustion of an IDF, the enrichment in oxygen means a lower oxidizer jet Reynolds number and thus less complete combustion occurs as a result of reduced amount of entrained ambient air.     

An experimental study of a cylindrical multi-hole premixed burner for the development of a condensing gas boiler

This study experimentally examined a cylindrical multi-hole premixed burner for its potential use for a condensing gas boiler, which produces less NO_x emissions and performs better. In this study, the hole diameters and the arrangement of a multi-hole burner were investigated using a flat burner model. The combustion characteristics for the flame stability as well as the NO_x and CO emissions were examined using a cylindrical burner. For an optimal operating condition, the equivalence ratio for the cylindrical burner was between 0.70 and 0.75. For this condition, the turn-down ratio was 3:1 or higher, which was suitable for appropriate control of the boiler operation. The NO_x and CO emissions were less than 40 ppm and less than 30 ppm, respectively, for a 0% O₂ basis. The LPG and LNG were able to be used in this type of burner because there was no phenomenal difference in the stable combustion region between them.     

NOx formation in post-flame gases from syngas/air combustion at atmospheric pressure

Species concentration measurements specifically those associated with nitrogen oxides (NO_x) can act as important validation targets for developing kinetic models to predict NO_x emissions under syngas combustion accurately. In the present study, premixed combustion of syngas/air mixtures, with equivalence ratio (Φ) from 0.5 to 1.0 and H₂/CO ratio from 0.25 to 1.0 was conducted in a McKenna burner operating at atmospheric pressure. Temperature and NO_x concentrations were measured in the post-combustion zone. For a given H₂/CO ratio, increasing the equivalence ratio from lean to stoichiometric resulted in an increase in NO and decrease in NO₂ concentration near the flame. Increasing the H₂/CO ratio led to a decrease in the temperature as well as the NO concentration near the flame. Based on the axial profiles above the burner, NO concentration increases right above the flame while NO₂ concentration decreases through NO₂-NO conversion reactions according to the path flux analysis. In addition, the present experiments were operated in the laminar region where multidimensional transport effects play significant roles. In order to account for the radial and axial diffusive and convective coupling to chemical kinetics in laminar flow, a multidimensional model was developed to simulate the post-combustion species and temperature distribution. The measurements were compared against both multidimensional computational fluid dynamics (CFD) simulations and one-dimensional burner-stabilized flame simulations. The multidimensional model predictions resulted in a better agreement with the measurements, clearly highlighting the effect of multidimensional transport.     

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.     

Operational characteristics of a parallel jet MILD combustion burner system

This study describes the performance and stability characteristics of a parallel jet MILD (Moderate or Intense Low-oxygen Dilution) combustion burner system in a laboratory-scale furnace, in which the reactants and exhaust ports are all mounted on the same wall. Thermal field measurements are presented for cases with and without combustion air preheat, in addition to global temperature and emission measurements for a range of equivalence ratio, heat extraction, air preheat and fuel dilution levels. The present furnace/burner configuration proved to operate without the need for external air preheating, and achieved a high degree of temperature uniformity. Based on an analysis of the temperature distribution and emissions, PSR model predictions, and equilibrium calculations, the CO formation was found to be related to the mixing patterns and furnace temperature rather than reaction quenching by the heat exchanger. The critical equivalence ratio, or excess air level, which maintains low CO emissions is reported for different heat exchanger positions, and an optimum operating condition is identified. Results of CO and NO_x emissions, together with visual observations and a simplified two-dimensional analysis of the furnace aerodynamics, demonstrate that fuel jet momentum controls the stability of this multiple jet system. A stability diagram showing the threshold for stable operation is reported, which is not explained by previous stability criteria.     

Influence of primary air ratio on flow and combustion characteristics and NOx emissions of a new swirl coal burner

Cold airflow experiments on a small-scale burner model, as well as in situ experiments on a centrally fuel-rich swirl coal combustion burner were conducted. Measurements were taken from within a 300 MW_e wall-fired pulverized-coal utility boiler installed with eight of centrally fuel-rich swirl coal combustion burners in the bottom row of the furnace during experiments. Various primary air ratios, flow characteristics, gas temperature and gas species concentrations in the burner region were measured. The results of these analyses show that with decreasing primary air ratio, the swirl intensity of air, divergence angles and maximum length and diameter of the central recirculation zone all increased, and the turbulence intensity of the jet flow peaked but decayed quickly. In the burner nozzle region, gas temperature, temperature gradient and CO concentration increased with decreasing primary air ratio, while O₂ and NO_x concentration decreased. Different primary air ratios, the gas temperatures and gas species concentrations in the side-wall region varied slightly.