The Effect of Diluent on the Sustainability of MILD Combustion in a Cyclonic Burner

The present study investigates the characteristics of MILD/flameless combustion in a cyclonic lab-scale burner. Such a configuration is effective for achieving turbulent mixing in a very short time while allowing for a reasonably long residence time for the development of combustion reactions. These two constraints are mandatory in the case of MILD combustion processes (high inlet temperatures and diluted mixtures). Such operating conditions are achieved through massive heat/mass recirculation towards the fresh incoming mixtures by recycling the exhausted gases, featuring a process where chemical kinetics times are elongated because of the dilution levels. Thus, long residence times are needed to achieve a satisfying reaction progress, and the high inlet temperatures result in fast and efficient mixing between disproportionate flows to avoid the onset of oxidation reactions before achieving diluted conditions. Under these constraints, a lab-scale facility was designed and built. The oxidation processes of C₃H₈/O₂ mixtures highly diluted in N₂ or CO₂ were investigated by varying the external parameters of the system, namely, the inlet temperature (up to 1300 K) and the mixture composition (from lean to rich mixtures). Several combustion regimes were experimentally identified. When the MILD regime was established, the combustion process became homogeneous within the burner without luminous emissions. To investigate the distributed nature of the MILD combustion processes, chemical simulations were performed under the assumption of a well-stirred reactor. For both the diluents, good agreement between the experimental and numerical results was obtained for MILD combustion conditions.     

Temperature response of turbulent premixed flames to inlet velocity oscillations

Flame–turbulence interactions are at the heart of modern combustion research as they have a major influence on efficiency, stability of operation and pollutant emissions. The problem remains a formidable challenge, and predictive modelling and the implementation of active control measures both rely on further fundamental measurements. Model burners with simple geometry offer an opportunity for the isolation and detailed study of phenomena that take place in real-world combustors, in an environment conducive to the application of advanced laser diagnostic tools. Lean premixed combustion conditions are currently of greatest interest since these are able to provide low NO _x and improved increased fuel economy, which in turn leads to lower CO₂ emissions. This paper presents an experimental investigation of the response of a bluff-body-stabilised flame to periodic inlet fluctuations under lean premixed turbulent conditions. Inlet velocity fluctuations were imposed acoustically using loudspeakers. Spatially resolved heat release rate imaging measurements, using simultaneous planar laser-induced fluorescence (PLIF) of OH and CH₂O, have been performed to explore the periodic heat release rate response to various acoustic forcing amplitudes and frequencies. For the first time we use this method to evaluate flame transfer functions and we compare these results with chemiluminescence measurements. Qualitative thermometry based on two-line OH PLIF was also used to compare the periodic temperature distribution around the flame with the periodic fluctuation of local heat release rate during acoustic forcing cycles.     

Influence of heat and mass transfer on the ignition and NOx formation in single droplet combustion

The effect of heat and mass transfer on the ignition, and in a second step on the nitrogen oxide (NO _x ) generation, of single burning droplets is examined in a numerical study. Spherical symmetry with no gravity and no forced convection is presumed; ambient temperature is set at 500 K, below the auto-ignition point. The essentials of a forced droplet ignition by an external energy source are introduced. Two methods are applied: heat introduction at a fixed radial position r and heat introduction at a fixed local equivalence ratio ϕ _r . This study’s distinctiveness compared to previous research is its focus on and its combination of partially pre-vaporized droplets and detailed chemistry, both being technically relevant in kerosene and diesel fuel combustion. The fuel of choice is n-decane (C₁₀H₂₂), and NO _x production is studied exemplarily as a representative group of pollutant emissions. The conducted simulations show a decrease of NO _x formation with an increase of the pre-vaporization rate \Uppsi. \Uppsi. This decrease is generally valid for both methods of heat introduction. However, results on flame stabilization and NO _x production reveal a high sensitivity to parameters of the ignition model. The burning behavior during the initial stages is dominated by the ignition position. Extracting heat from the exhaust gas region of burning droplets shows no impact on the flame position nor on the relative NO _x production. As a consequence, a well-founded modeling of the investigated droplet regime needs to resort to an iterative adaptation of the heat introduction parameters based on the findings of droplet burning and exhaust gas production.     

H2O and CO2 Dilution in MILD Combustion of Simple Hydrocarbons

MILD combustion is a very attractive technology because of its intrinsic features for energy production from diluted gas deriving from bio- or thermochemical degradation of biomass. An effective use of such a technology for diluted fuel requires a thorough analysis of ignition and oxidation behavior to highlight the potential effects of the different fuel components on the basis of temperature and diluent/oxygen/fuel mixture composition. In this work, ignition and oxidation of a model gas surrogate for the gaseous fraction of biomass pyrolysis products containing C₁-C₂ species, CO and CO₂ were experimentally and numerically studied over a wide range of temperature and overall composition in the presence of large amounts of CO₂ or H₂O. Experimental results showed that such species significantly alter the evolution of the ignition process in dependence on temperature range and mixture composition. Several kinetic models were tested to simulate experimental results. Significant discrepancies occur, especially in the case of steam dilution. Numerical analyses suggested that such diluents acted mainly as third body species at low temperatures, conditioning both radical production pathways and the relative weight of C₁ oxidation/recombination routes, while strongly interacting with the H₂/O₂ high temperature branching mechanisms at high temperatures. Further analyses are mandatory to improve the predictability of the models and extend the applicability of the chemical schemes to non-standard conditions.     

Investigations of the reduction of NO to N2 by reaction with Fe under fuel-rich and oxidative atmosphere

The limitation of nitrogen oxides (NO_x) emissions from stationary combustion chambers is still an important issue in the field of the natural environment protection. This paper describes the reduction of NO_x in the presence of iron. A few new aspects of research describing the utilization of iron as an additive that influences NO_x reduction at high temperatures are presented. In particular, the influence of the excess air number (λ) on the NO_x removal efficiency in the presence of Armco iron was determined. A >50 % increase in the efficiency was achieved at λ = 0.6. The research was conducted using N₂/NO mixed with a flue gas from carbon monoxide combustion. In addition, a correction factor (Arrhenius-type empirical equation) was determined, which enabled the calculation of the oxygen influence on the inhibition of the de-NO_x reaction in the presence of iron. The NO_x reduction was also tested using bearing steel samples. Finally, the use of iron in de-NO_x processes under different combustion conditions is briefly reviewed and analyzed.     

WRF/CHEM modeling of impacts of weather conditions modified by urban expansion on secondary organic aerosol formation over Pearl River Delta

In this paper, the online Weather Research and Forecasting and Chemistry （WRF/CHEM） model, coupled with urban canopy （UCM） and biogenic-emission models, is used to explore impacts of urban expansion on secondary organic aerosols （SOA） formation. Two scenarios of urban maps are used in WRF/CHEM to represent early 1990s （pre-urbanization） and current urban distribution in the Pearl River Delta （PRD）. Month-long simulation results using the above land-use scenarios for March 2001 show： （1） urbanization can increase monthly averaged temperatures by about 0.63 ℃, decrease monthly averaged 10-m wind speeds by 38%, increase monthly averaged boundary-layer depths by 80 m, and decrease monthly aver- aged water mixing ratio by 0.2g/kg. （2） Changes in meteorological conditions can result in detectable concentration changes of NOx, VOC, O3 and NO3 radicals. Urbanization decreases surface NOx and VOC concentrations by a maximum of 4 ppbv and 1.5 ppbv, respectively. Surface O3 and NO3 radical concentrations over major cities increase by about 2-4 ppbv and 4-12 pptv, respectively; areas with increasing O3 and NO3 radical concentrations generally coincide with the areas of temperature increase and wind speed reduction where NOx and VOC decrease. （3） Urbanization can induce 9% increase of SOA in Foshan, Zhongshan and west Guangzhou and 3% decrease in Shenzhen and Dongguan. Over PRD major cities, SOA from Aitken mode reduces by 30% but with more than 70% SOA from accumulate mode. Urbanization has stronger influence on SOA formation from Aitken mode. （4） Over the PRD, 55-65% SOA comes from aromatics precursors. Urbanization has strongest influence on aromatics precursors to produce SOA （14% increase）, while there is less influence on alkane precursors. Alkene precursors have negative contribution to SOA formation under urbanization situation.     

Numerical investigation of full scale coal combustion model of tangentially fired boiler with the effect of mill ducting

In this paper a full scale combustion model incorporating upstream mill ducting of a large tangentially fired boiler with flue gas recirculation was examined numerically. Lagrangian particle tracking was used to determine the coal particle paths and the Eddy Dissipation Model for the analysis of the gas phase combustion. Moreover volatiles and gaseous char products, given off by the coal particles were modelled by Arrhenius single phase reactions and a transport equation was solved for each material given off by the particles. Thermal, prompt, fuel and reburn NO _x models with presumed probability density functions were used to model NO _x production and the discrete transfer radiation model was used to model radiation heat transfer. Generally, the findings indicated reasonable agreement with observed qualitative and quantitative data of incident heat flux on the walls. The model developed here could be used for a range of applications in furnace design and optimisation of gas emissions of coal fired boiler plants.     

Porous Burner for Application in Stationary Gas Turbines: An Experimental Investigation of the Flame Stability, Emissions and Temperature Boundary Condition

A possible solution to ensure the stability of lean premixed flames over an extended operational range is to provide enhanced heat recirculation by employing porous inert material. A potential application of the porous burner concept is the generation of the pilot flames based on lean premixed combustion which is a prerequisite for ultra low NO_x emission. For the optimization of the porous burner an experimental study investigating flame stability and emissions was conducted. In particular axial concentration profiles of the stable species and temperature within the porous burner reaction zone are presented. Furthermore the surface temperature of the burner having a 10 PPI SiSiC material was measured for various operating conditions using two colour pyrometry.     

Modeling of Turbulent Natural Gas and Biogas Flames of the Delft Jet-in-Hot-Coflow Burner: Effects of Coflow Temperature,Fuel Temperature and Fuel Composition on the Flame Lift-Off Height

The structure of a turbulent non-premixed flame of a biogas fuel in a hot and diluted coflow mimicking moderate and intense low dilution (MILD) combustion is studied numerically. Biogas fuel is obtained by dilution of Dutch natural gas (DNG) with CO₂. The results of biogas combustion are compared with those of DNG combustion in the Delft Jet-in-Hot-Coflow (DJHC) burner. New experimental measurements of lift-off height and of velocity and temperature statistics have been made to provide a database for evaluating the capability of numerical methods in predicting the flame structure. Compared to the lift-off height of the DNG flame, addition of 30 % carbon dioxide to the fuel increases the lift-off height by less than 15 %. Numerical simulations are conducted by solving the RANS equations using Reynolds stress model (RSM) as turbulence model in combination with EDC (Eddy Dissipation Concept) and transported probability density function (PDF) as turbulence-chemistry interaction models. The DRM19 reduced mechanism is used as chemical kinetics with the EDC model. A tabulated chemistry model based on the Flamelet Generated Manifold (FGM) is adopted in the PDF method. The table describes a non-adiabatic three stream mixing problem between fuel, coflow and ambient air based on igniting counterflow diffusion flamelets. The results show that the EDC/DRM19 and PDF/FGM models predict the experimentally observed decreasing trend of lift-off height with increase of the coflow temperature. Although more detailed chemistry is used with EDC, the temperature fluctuations at the coflow inlet (approximately 100K) cannot be included resulting in a significant overprediction of the flame temperature. Only the PDF modeling results with temperature fluctuations predict the correct mean temperature profiles of the biogas case and compare well with the experimental temperature distributions.     

A review of recent advances in selective catalytic NOx reduction reactor technologies

Research on NO_x treatment is extensive in recent years due to growing environmental awareness. Selective catalytic reduction (SCR) of NO_x, as a proven technology, offers higher NO_x control efficiency than many other NO_x treatment methods. The present work reviews the recent development of SCR reactor technologies. Firstly, catalysts and mechanism of different SCRs were briefly summarized. Different SCR reactors, e.g. structured reactor, fluidized bed reactor and moving bed reactor, were then discussed. As a more advanced technology, multifunctional reactors were also developed for SCR process and could be divided into two categories: decoupled adsorption-reaction process and combined SCR system. The mechanism and properties of these processes were discussed in detail. Some recommendations were given for the future work in SCR reactor design. SCR reactor technology for emerging energy processes was also addressed, such as oxyfuel combustion and biofuel conversion processes, which put forward new requirements for SCR technologies and also open new opportunities for advanced design of SCR reactors.     

Experimental Characterization of Premixed Flame Instabilities of a Model Gas Turbine Burner

In recent years, the NO _x emissions of heavy duty gas turbine burners have been significantly reduced by introducing premixed combustion. These highly premixed burners are known to be prone to combustion oscillations. In this paper, investigations of a single model gas turbine burner are reported focusing on thermo-acoustic instabilities and their interaction with the periodic fluctuations of the velocity and pressure. Phase-locked optical measurement techniques such as LDA and LIF gave insight into the mechanisms.Detailed investigations of a gas turbine combustor rig revealed that the combustor as well as the air plenum oscillate in Helmholtz modes. These instabilities could be attributed to the phase lag of the pressure oscillations between the air plenum and the combustor, which causes an acceleration and deceleration of the air flow through the burner and, therefore, alternating patterns of fuel rich and lean bubbles. When these bubbles reach the reaction zone, density fluctuations are generated which in turn lead to velocity fluctuations and, hence, keep up the pressure oscillations.With increasing the equivalence ratio strong combustion oscillations could be identified at the same frequency. Similarly as with weak oscillations, Helmholtz mode pressure fluctuations are present but the resulting velocity fluctuations in the combustor can be described as a pumping motion of the flow. By the velocity fluctuations the swirl stabilization of the flame is disturbed. At the same time, the oscillating pressure inside the combustor reaches its minimum value. Shortly after the flame expands again, the pressure increases inside the combustor. This phenomenon which is triggered by the pressure oscillations inside the air plenum seems to be the basic mechanism of the flame instability and leads to a significant increase of the pressure amplitudes.     

Burning of single carbon particle approached by activation energy asymptotics

The method of activation energy asymptotics is used to treat the combustion of a single carbon particle in quiescent gas mixture with high temperature. Both heterogeneous reactions 2C+O₂ 2CO, C+CO₂ 2COand homogeneous reaction 2CO+O₂ 2CO₂ are considered. It is shown that the burning of the particle principally is carried out during a diffusion-limited period. Four brief and complex periods through which the history of the particle evolves from a heat-up period to the diffusion-limited period are described. A comparison between results of activation energy asymptotics and exact numerical solutions is given. The agreement is considered satisfactory.     

Tailoring V2CTx to form a derivative hybrid by partial oxidation for enhanced lithiation behavior

Utilizing MXene to form the multifunctional derivative is a route to construct high-performance electrode materials. To address this issue, V₂CT_x MXene was employed to realize a derivative of VO_x/V₂CT_x MXene via a partial oxidation process. Relying on the annealing in the air atmosphere, the controlled oxidation behavior transformed V₂CT_x MXene partially to V₂O₅ and formed a derivative hybrid of V₂O₅/V₂CT_x MXene. As a result, a package of capacity, rate performance, and cyclability can be enhanced. This work explores the derived behavior of MXenes and provides a route for constructing the hybrid with less interface contact. Furthermore, these findings can be extended to other MXene materials.     

Optical investigation of the combustion behaviour inside the engine operating in HCCI mode and using alternative diesel fuel

In order to understand the effect of both the new homogeneous charge compression ignition (HCCI) combustion process and the use of biofuel, optical measurements were carried out into a transparent CR diesel engine. Rape seed methyl ester was used and tests with several injection pressures were performed. OH and HCO radical were detected and their evolutions were analyzed during the whole combustion. Moreover, soot concentration was measured by means the two colour pyrometry method. The reduction of particulate emission with biodiesel as compared to the diesel fuel was noted. Moreover, this effect resulted higher increasing the injection pressure. In the case of RME the oxidation of soot depends mainly from O₂ content of fuel and OH is responsible of the NO formation in the chamber as it was observed for NO_x exhaust emission. Moreover, it was investigated the evolution of HCO and CO into the cylinder. HCO was detected at the start of combustion. During the combustion, HCO oxidizes due to the increasing temperature and it produces CO. Both fuels have similar trend, the highest concentrations are detected for low injection pressure. This effect is more evident for the RME fuel.     

Multiyear analyses of ground-level air contaminants over Paris metropolitan region using real-time observations and air mass backward trajectories

For the years 2008–2013, particles of diameter <10 and 2.5 μm (PM₁₀ and PM_2.5, respectively), NO_x, SO₂, and O₃ concentrations at urban, suburban, rural, and traffic sites in the Paris metropolitan area were analyzed. Strong spatial variability at traffic and rural sites and relatively uniform profiles at urban and suburban sites for PM₁₀, PM_2.5, and O₃ were observed. The O₃ weekend effect was induced by lower NO_x emissions during the weekend, and favored volatile organic compounds (VOCs)-limited atmospheric conditions. In conjunction with low ambient temperature, these conditions could also favor increased formation of secondary particulate nitrates in winter. Winter air pollution events were associated with multiple pollutants, whereas those observed in spring were caused by high PM₁₀ and PM_2.5 levels. Backward trajectory analyses showed the contribution of sources in Western and Central Europe on days with high PM₁₀, PM_2.5, and O₃, and a local/national component for NO_x and SO₂.     

Design and operation assessment of an oxyfuel fluidized bed combustor

Oxyfuel combustion is a promising alternative for CO₂ capture. While this has been proven in pulverized fuel (PF) burners, research in fluidized bed (FB) reactors is still scarce. Our work aims to increase the knowledge about this technology. To this purpose, a 95 kW_th FB oxyfuel combustion test rig has been erected. Its main characteristics are described in this paper, giving detailed information on the subsystems: the FB reactor, the fuel and oxidant supplies, and ancillaries. Plant flexibility is emphasized. It allows to operate under different CO₂/O₂ ratios, and to recycle CO₂ from the flue gases. Both the processes design and monitoring are supported by simulations that have been validated against experimental data, regarding fluid dynamics, combustion, and heat transfer. Finally, the performance of the facility has been tested both with coal alone and blended with biomass. CO₂ concentrations over 90% (dry basis) in the flue gases have been obtained. Comparison of air and oxygen combustion tests and operational recommendations are discussed, confirming the feasibility of the FB oxyfuel technology for CO₂ capture purposes.     

An Experimental Comparison of the Emissions Characteristics of Standard Jet A-1 and Synthetic Fuels

Emissions characteristics of lean, turbulent, partially premixed swirled flames of synthetic fuels along with a standard Jet A-1 fuel are studied. The investigated synthetic fuels are (a) Fully synthetic jet fuel (FSJF), (b) Fischer Tropsch synthetic paraffinic kerosene (FT-SPK), (c) FT-SPK+20 % hexanol, and (d) FT-SPK+50 % naphthenic cut. The measurements are performed in a tubular combustor equipped with a burner based on the principle of air-blast atomization. The exhaust gas compositions are measured using a non-dispersive infrared gas analyzer for carbon dioxide (CO₂) and carbon monoxide (CO), a flame ionization detector for unburned hydrocarbons (UHC), and a chemical luminescence detector for nitric oxides (NO and NO₂). The emissions indices (EI) of CO and NO_X of the investigated fuels are calculated using guidelines provided by the Society of Automotive Engineers (SAE). Measurements are performed at several combustor pressure levels, i.e., 0.3, 0.54 and 0.8 MPa, to compare the emissions behavior of the investigated fuels at varied operating conditions. At 0.3 MPa of combustor pressure, the order of fuels with their increasing formation of NO_X are FSJF, FT-SPK+20 % hexanol, Jet A-1, FT-SPK+50 % naphthenic cut and neat FT-SPK. Differences in the observed NO_X formation behavior of the investigated fuels are attributed to their probable different degrees of mixing with air in the combustor. At 0.8 MPa, no significant differences in their emissions characteristics are observed due to very low absolute values; hence we report that at higher pressure conditions which prevail in the aero-engine combustion systems, the emissions characteristics of tested synthetic fuels are very close to that of standard Jet A-1 fuel.     

Coupling tabulated chemistry with Large Eddy Simulation of turbulent reactive flows

A new modeling strategy is developed to introduce tabulated chemistry methods in the LES of turbulent premixed combustion. The objective is to recover the correct laminar flame propagation speed of the filtered flame front when the subgrid scale turbulence vanishes. The filtered flame structure is mapped by 1D filtered laminar premixed flames. Closure of the filtered progress variable and the energy balance equations are carefully addressed. The methodology is applied to 1D and 2D filtered laminar flames. These computations show the capability of the model to recover the laminar flame speed and the correct chemical structure when the flame wrinkling is completely resolved. The model is then extended to turbulent combustion regimes by introducing subgrid scale wrinkling effects on the flame front propagation. Finally, the LES of a 3D turbulent premixed flame is performed. To cite this article: R. Vicquelin et al., C. R. Mecanique 337 (2009).