Olivier Charon

Soot and NO formation in methane–oxygen enriched diffusion flames

NO and soot formation were investigated both numerically and experimentally in oxygen-enriched counterflow diffusion flames. Two sets of experiments were conducted. In the first set, the soot volume fraction was measured as a function of oxygen content in the oxidizer jet at constant strain rate (20 s−1). In the second set of experiments, the soot volume fraction was measured as a function of strain rate variation from 10 to 60 s−1 and at constant oxygen content on the oxidizer side. A soot model was developed based on a detailed C6 gas phase chemistry. The soot and molecular radiation were taken into account. Numerical results were verified against experimental data. The soot volume fraction was predicted with the maximum discrepancy less than 30% for all cases considered. It was found that oxygen variation significantly modified the diffusion flame structure and the flame temperature, resulting in a substantial increase of soot. The temperature increase promotes aromatics production in the fuel pyrolysis zone and changes the relative contributions of the thermal and Fenimore mechanisms into NO formation. As the strain rate increases, the residence time of incipient soot particles in the high temperature zone is reduced and the total amount of soot decreases. High concentration of soot in the flame leads to enhancement of radiant heat exchange: the reduction of temperature due to radiation was found to be between 10 and 50 K. This caused a reduction of peak NO concentrations by 20%–25%. The increase of oxygen content in the oxidizer stream resulted in a reduction of the distance between the plane of the maximum temperature and the stagnation plane.

Soot formation effects of oxygen concentration in the oxidizer stream of laminar coannular nonpremixed methane/air flames

Abstract This experimental investigation analyzes the soot formation effects of oxygen concentration in the oxidizer stream (O 2 + N 2 ) ventilating laminar jet nonpremixed methane flames. The base flame incorporates air as the oxidizer; two additional flames, with respective oxygen concentrations of 50% and 100% in the ventilating coflow, are also examined. The microstructure of soot collected from selected flame locations is determined combining thermophoretic sampling and transmission electron microscopy. A laser-light extinction technique is employed along with tomographic inversion to measure the soot volume fraction distributions within the three flames. The results indicate that soot surface growth and oxidation rates in the methane/50% oxygen flame are higher compared to the respective rates in the methane/air base flame. The rate of soot inception becomes stronger with increasing oxygen content in the oxidizer stream. Soot yields diminish with increasing oxygen concentration, as do luminous flame spatial dimensions. Soot aggregate data on the soot annulus suggest a higher degree of agglomeration under oxygen-enriched conditions. Finally, the fractal dimensions of selected soot aggregate samples are measured to be 1.64 (methane/air flame) and 1.65 (methane/50% oxygen flame), being similar to previously published values for carbonaceous soot.

Simulating the Impact of Oxygen Enrichment in a Cement Rotary Kiln Using Advanced Computational Methods

This work presents the simulation of a rotary kiln used to produce cement clinker. The effort uses an original approach to kiln operation modeling. Thus, the moving cement clinker is accurately simulated, including exothermal reactions into the clicker and advanced heat transfer correlations. The simulation includes the normal operation of a cement kiln, using coal in an air-fired configuration. The results show the flame characteristics, fluid flow, clinker and refractory characteristics. Two types of coal are employed, one with medium-volatile and one with low-volatile content, with significant differences noted in the kiln operation. A specific goal of this effort is to study the impact of oxygen enrichment on the kiln operation. For this purpose, oxygen is lanced into the kiln at a location between the load and the main burner, and the impact of oxygen enrichment on the kiln operation is assessed. Different oxygen injection schemes are also studied. Thus, varying the angle of the oxygen lance enables to handle various problems as reducing conditions, overheating in the burning zone or refractory wall. It is concluded that oxygen has a beneficial role in the fuel combustion characteristics, and its impact on refractory temperature and the clinker is negligible, in conditions of increased productivity and overall efficiency. The paper presents the impact of dust insufflation into the kiln, such as reduced temperature profile, resulting in a less stable combustion process. The work shows the beneficial influence of oxygen enrichment on kiln operation in the presence of dust, leading to an increase in the amount of dust capable of being insufflated into the kiln. The paper presents the impact of dust insufflation into the kiln, such as reduced temperature profile, resulting in a less stable combustion process. The work shows the beneficial influence of oxygen enrichment on kiln operation in the presence of dust, leading to an increase in the amount of dust capable of being insufflated into the kiln.

Multifunctional industrial combustion process monitoring with tunable diode lasers

12 To address the inherent issues with extractive sampling, Air Liquide and PSI are collaborating on the development of an in-situ multi-functional near-IR tunable diode laser system. The system is specifically targeted for application in harsh combustion environments with flue gas temperatures > 1600 degree(s)C and high particle densities. The multiplexing capability of the diode laser system allows near simultaneous detection of CO, O2, and H2O. These are essential species in characterizing the combustion state of the process, i.e., fuel-rich or fuel-lean, and the flue gas temperature. Sensor development and testing are conducted on a 700 kW oxy-fuel pilot furnace to evaluate the performance under simulated industrial conditions. Here we present pilot test results for dynamic stoichiometry changes, effect of particle entrainment, and air infiltration monitoring.

Pulverised Coal Combustion Under Transient Cloud Conditions in a Drop Tube Furnace

Abstract A low volatile coal was burned with oxygen-enriched air in a drop tube furnace at 1223 and 1523 K, either as single particles or as small batches added as a pulse. The particle temperatures and burnout times at the two furnace temperatures were recorded for each test. The ignition / combustion processes were recorded by pyrometer and video camera. The pulsed batch burnout times were ten to twenty times longer than the single particle times, indicating that cloud combustion was taking place. A mathematical model of the transient behaviour of the batch combustion process was developed, based on plug flow. Effects arising from volatile combustion were apparent in the visualisations but were ignored in the modelling. Burning was predominantly under diffusion control especially at high oxygen concentrations. The effect of particle segregation due to feeder performance and aerodynamic drag was simulated. From the dispersion of the particles, values of the local cloud combustion number G during burnout were determined. The presence of cloud combustion during most of the burning process was confirmed. In a drop tube furnace the mass transfer process supplying oxygen to the interior of the particle cloud is convection brought about by the slip between the particles and the gas. A new simplified cloud combustion number Gi, which combines convection and reaction kinetics was developed for this situation. Gi can then be related to the effectiveness factor for combustion rate in the cloud. The new approach predicts that serious oxygen depletion will arise during the bumout of the cloud, as is observed in practice. The cloud number Gi was successful in predicting the cloud bumout times from the single particle values.

Pulsated O2/fuel flame as a new technique for low NOx emission

ABSTRACT A new primary NOx reduction method is described. It consists in pulsating one or both fluids from a oxy-gas burner by rotary-plug valve or solenoid electrovalve. The parameters to be set are C the constant fraction of the pulsated gas flow-rate, P the amplitude of the pulsated fraction of the gas flow-rate, O the duration of the maximum flow-rate, F the duration of the minimum flow-rate and the dephasing between the two pulsations when both gases are pulsated. Experiments on 20 kW and 50 kW pilot furnaces have shown that up to 90% NOx reduction can be achieved without lowering heat transfer when parameters are set as follows (both gases are pulsated): dephasing equal to 1/2, C/P around 0.2 and frequency (1/F+O)) as low as possible. Further developments are underway to perform test on a 1 MW pilot furnace before industrial trials

Industrial combustion monitoring using optical sensors

With stricter environmental regulations optimization of the combustion process for reduced pollutant emission and higher fuel efficiency is a major objective for manufacturers. The promotion of oxy-fuel combustion is one alternative technology that has been demonstrated as a means for manufacturers to meet their environmental objectives. Despite the benefits oxy-fuel combustion can offer further optimization using monitoring and control techniques are still needed. Here we present a novel method for monitoring and controlling oxy-fuel burners by strategic placement of optical sensors. The sensors are integrated into an industrial oxy-fuel capable of withstanding harsh environments. Radiation from the flame at selected wavelength regions is collected by fiber optics attached to the burner and transported to a miniaturized PC-based spectrometer. The spectral information obtained is used to construct a neural network (NN) model that relates the real- time signal collected to burner operating parameters such as, stoichiometry, power, and fueled and/or oxidizer changes. This processed information from the NN can then be used in a control-loop for adjusting and optimizing combustion parameters or alerting operators of potential burner problems. Examples of using this technology on AIr Liquides pilot furnaces in both the US and France and from an industrial glass melting tank will be presented. The potential of the sensor and NN approach is demonstrated for both conventional burner and an advanced wide flame burner. The results show that both stoichiometry and power changes can reliably be detected by use of the optical sensors. In addition, an example demonstrating the method on oxy-fuel oil flames to monitor oil atomization quality and stoichiometry will be presented.

Modified sodium line-reversal temperature measurements in oxy-fuel flames

Oxy-fuel technology that uses high purity oxygen in place of air has demonstrated to be a cost-effective method for improving melting operations providing benefits in fuel savings, reduction in capital investment, and reduction of NOx and particulate matter. These benefits are evident in the glass industry where an estimated 15 percent of the US production has already been converted to oxy-fuel. However conversion from air-fuel is complicated by the drastic differences between the combustion characteristics such as flame temperature, momentum, flame chemistry, and heat transfer properties. For optimum performance using oxy-fuel combustion well-characterized burners with knowledge of the temperature in the combustion space is needed. Temperature characteristics for a given burner design are useful for both validation and parameter adjustment in 3D numerical models and optimizing the flame to the process. Because of the higher temperatures and steeper gradients in oxy-fuel flames traditional measurement techniques used on industrial flames, e.g., suction pyrometer or coherent anti-stokes Raman spectroscopy have limited use. Here we present results using a modified line reversal technique to monitor the emission and transmission of oxy-fuel flames seeded with sodium. The technique provides real-time information on the line-of-sight temperature observed from industrial scale turbulent flames.

Oxy-fuel combustion emission monitoring using tunable diode laser sensors

With stricter environmental regulations, optimization of the combustion process for reduced pollutant emission and higher fuel efficiency has become an industry objective. To achieve these objectives, continuous monitoring of key processes parameters such as temperatures, fuel and oxidant input, and flue gas composition is required. For flue gas composition monitoring conventional extractive sampling techniques are typically used. However these techniques suffer from slow response time due to long sample lines and are sensitive to plugging problems when applied to particle-laden flows. Using in-situ monitoring with near-IR tunable diode lasers (TDL) eliminates the problems encountered with extractive sampling. The chemical species to be monitored dictates the wavelength range of the diode lasers used. These lasers are rapidly tuned over an absorption line to obtain concentration along the line-of-sight path. In addition, gas temperature can be measured by scanning the laser over multiple rotational lines of a target molecule. Here we demonstrate the feasibility of using TDLs for in-situ O2 monitoring on the exhaust end of Air Liquides oxy-fuel pilot furnace. Tests were conducted at various operating conditions and compared with conventional extractive sampling measurements. The response time of the technique is demonstrated by measurements conducted on a dynamic system where the fuel flow is oscillated at low frequencies. In addition, to study the effect of dirty gas streams typically found on industrial processes, seed particles were introduced into the burner to simulate particle-laden flows.