Mohammed Pourkashanian

Experimental and modelling study of sulfur and nitrogen doped premixed methane flames at low pressure

Laser-induced fluorescence (LIF) has been used to observe NS and NO in methane/oxygen/argon laminar flames at low pressure doped with ammonia and sulfur dioxide. NS profiles as a function of height above the burner have been measured for rich flames. The effect of adding various amounts of sulfur dioxide on the observed NO in the burnt gas region has been investigated for a variety of stoichiometries. The experimental measurements have been compared with PREMIX simulations using a detailed elementary reaction mechanism for nitrogen- and sulfur-containing species in a methane flame. Sensitivity analysis has been employed to highlight the important reactions for NS, NO and SO2. The results demonstrate significant uncertainties in currently best available rate data for important reactions involving sulfur-containing species.

NO x Formation in Large-Scale Oxy-Fuel Flames

The use of pure oxygen in the combustion of fossil-fuels is fuelled by the large potential for NO x reduction. This paper describes experiments conducted on a semi-industrial scale test furnace using a single oxy-natural gas burner. The burner was operated in two modes : the oxygen acting as the high velocity fluid, entraining the fuel gas into the flame, and vice versa. A steam-cooled gas-sampling probe was employed allowing gas samples to be taken from inside the furnace. Temperature and species concentration, including NO x were measured and were corroborated by numerical predictions. This study investigated two main areas, (a) the influence of mixing on the flame characteristics and the formation of nitric oxide in semi-industrial scale burners at high temperature combustion and (b) verification of the model used for high temperature combustion processes. The numerical analysis indicates that reasonable agreement can be obtained for the gas temperature and species concentration, using a 4-step reaction mechanism for the combustion process. The temperature predictions appear to be accurate within ± 50 K and computed NO emissions differ at most by 20% from measured NO x levels. This study shows that NO x emissions are influenced by the mixing of oxygen and fuel and injection angles. Also fast combustion produced a higher flame temperature and complete combustion but resulted in lower NO emissions.

Modeling the reaction of oxygen with coal and biomass chars

The combustion of coal is responsible for nearly 40% of the worlds electricity production, and char combustion accounts for about half of that amount. Clearly, an understanding of the combustion mechanism of carbon is of great importance not only because of its industrial significance but because it is a model heterogeneous reaction. A number of recent studies have been concerned with ab initio molecular orbital calculations on graphite including model chemistry and the reactions with molecular oxygen. This study is concerned with oxidation steps involving the attachment of oxygen to a graphene layer at high temperature leading to the formation of carbon monoxide, and particular attention is paid to the subsequent oxidation reactions. In addition, the reaction of oxygen with carbon catalyzed by metals inherent within the char matrix and the reaction of molecular oxygen with the analogous biomass char are investigated and their reaction paths are discussed.

The Prediction of NOx Emissions from Spray Combustion

ABSTRACT The study of in-flame chemical processes involving nitrogenous species forms an important pan of the understanding of the design parameters which lead to lower NOx emissions from combustion systems. This paper presents data from experimental and modelling studies on the nitrogenous emissions from an oil fired furnace using staged combustion for the control of NOx emissions. Measurements of the in-flame NO concentration profiles are compared for the same burner operating in the unstaged and staged mode. The exit gas NO concentration was reduced by 30% during staged combustion with 35% secondary air. A post-processing NO model linked to the output from a commercial computational code was used to predict the rates of formation and concentrations of thermal, fuel and prompt-NO from the experimental system. The predictions showed that NO emission is in good qualitative agreement with the experimentally observed values when the effects of superequilibrium radical formation and turbulence/chemistry inte...

Modelling the formation and emission of environmentally unfriendly coal species in some gasification processes

Abstract During coal gasification the environmentally unfriendly components, sulfur, nitrogen, chlorine and the trace metals, can be released with the tar and gaseous species or retained in the carbonaceous char and ash. The fate of these components is dependent on the type of coal, on the reaction conditions to which they are subjected during gasification, on downstream pollution control devices and on the type of gas turbine used for electricity production. A model has been developed to predict the fate of these components under a range of conditions, so that suitable measures can be devised to minimize their environmental impact, and to estimate their effects for two generic types of gasifier, namely those based on air and on oxygen. The trace metal distribution could be modelled accurately up to 1100°C but uncertainty increased at higher temperatures. Ammonia and hydrogen cyanide were found to be mainly formed from the coal nitrogen and their pathways were followed by kinetic modelling. These compounds can affect the level of NO x produced by the gas turbine.

Inadequacy of Optical Smoke Measurements for Characterization of Non–Light Absorbing Particulate Matter Emissions from Gas Turbine Engines

Analysis of particulate matter (PM) emissions from gas turbine engines, using the conventional smoke number (SN) technique, provides a measure of plume visibility. In this study, PM emissions were sampled from the exhaust of a small gas turbine engine, burning Jet A-1, and Biodiesel. SN results indicated that biodiesel significantly reduced visible emissions. Analysis of PM number and mass concentrations using a differential mobility spectrometer found that although nonvolatile PM was significantly reduced, biodiesel combustion produced a high fraction of volatile PM. Concurrent aerosol mass spectrometer measurements established that the condensable material was organic in composition. The condensation of volatile organics, not captured by the SN technique, significantly increased the total PM emissions. Application of the Society of Automotive Engineers Aerospace Recommended Practice 1179d for gas turbine engines is limited to visible plume characterization and thus is inadequate when combustion produces a large fraction of volatile or non–light absorbing PM emissions.

CFD Predictions of Single Row Film Cooling With Inclined Holes: Influence of Hole Outlet Geometry

A CFD investigation of a single row of round inclined film cooling holes in a crossflow has been carried out with the view of investigating the discrepancies in the literature between predicted and measured results. The experimental results of Sinha et al. [1], Kohli et al. [40], Pedersen et al. [3] and others form the data base for validation of the CFD prediction of film cooling. Previous work in the literature is reviewed to show that CFD has had difficulty in obtaining agreement with these basic experimental film cooling results. However, most previous work has used tetrahedral meshes which gave poor agreement with experiments in the near hole region. In the present work it is shown by direct comparison of tetrahedral and hexahedral meshes, using the FLUENT code, with the same turbulence models, that only hexahedral meshes give good agreement with the experimental results in the near hole region. It is postulated that the reason is that the mesh structure is aligned with the flow and has more computational nodes in the important film cooling boundary layer. The hexahedral mesh was used with five turbulence models, which showed the standard k-epsilon model consistently gave the best agreement with experimental data for narrow angle film cooling. This CFD methodology was shown to be capable of predicting the influence on film cooling effectiveness of trench hole and larger diameter outlet hole geometries.Copyright

CFD Predictions of Low NOx Radial Swirlers With Vane Passage Fuel Injection With Comparison With Internal Gas Analysis Flame Composition

Radial swirlers with vane passage natural gas injection, similar to those used in some industrial low NOx gas turbines, were investigated for their flame structure both experimentally and using CFD. The radial swirler NOx emissions at 600K and 1 atmosphere pressure were shown to be 3–4 ppm at 15% oxygen at 1800K and 1–2 ppm at 1700K. These levels were similar to the best published low NOx emissions using any flame stabilizer design. A flame at O = 0.5 and 600K air temperature was investigated for its structure using a 10mm OD water cooled gas sample probe with a 1mm gas sample inlet on the upstream side of the probe. This showed that the mixing in the vane passage and outlet duct was very good. The maximum unmixedness at the first traverse location, 10mm downstream of the dump expansion zone, was 20% of the mean and the unmixedness was less than 5% within 30mm from the dump expansion. The flame structure was shown to involve a thick turbulence reaction zone of about 100mm thickness to the 90% heat release point. The CFD predictions were made using the RSM and k-e turbulence models and the flamelet combustion model with a strain rate library. The isothermal aerodynamics predictions were in good agreement with others for similar geometries. There was an inner and outer recirculation zone with a swirling shear layer between. The peak turbulent kinetic energy was predicted to be on the inside of the shear layer. The experimental results showed that the flame developed in this region of high turbulence and low axial velocities. The flamelet model was less successful at predicting the flame development. The NOx results were predicted to be 2ppm less than the experimental results, due to the shorter predicted heat release region with associated lower prompt NOx.Copyright

FUNDAMENTAL STUDIES OF A PARTIAL PREMIXED COUNTER-FLOW COMBUSTION SYSTEM AND ITS EFFECT ON NOx EMISSIONS

The use of air staging as a technique for reducing NOX emissions from process industry burners has been employed for many years but lower levels of NOX and high thermal efficiencies can be achieved using a partial premixed combustion mode. The development of partial premixed burners has only gradually increased because of the increased risk of flash back and the difficulties of control. The current study presents both an experimental and a numerical investigation on a laboratory-scale counter-flow combustion system of the partial premixed mode. The system aims to duplicate the mixing process observed within an industrial burner, which produces a double flame structure. Results have shown that the nozzle configuration has a direct influence on NOX emissions.

Predictions of Effusion Cooling With Conjugate Heat Transfer

This work involves CFD conjugate heat transfer modelling of the geometrical design influence on effusion cooling. Experimental data was modelled for the overall effusion film cooling effectiveness using Nimonic 75 walls with imbedded thermocouples. The Fluent CFD code was used to investigate the experimental configuration for a 10×10 square array of holes with a 90° injection angle. In the computational predictions, 10000ppm of methane tracer gas was added to the coolant and the concentration at the wall allowed the adiabatic cooling effectiveness of the effusion film cooling to be predicted separately from the overall wall cooling effectiveness. The predicted overall cooling effectiveness results show that the wall was locally at a uniform temperature, but the axial development of the cooling film does result in a gradual reduction of the wall temperature with axial distance. The predictions show that the heating of the coolant by the hot wall was equally split between the hole approach flow on the backside of the wall and inside the film cooling holes. This heating changed the conditions in the film cooling layer from those of the equivalent adiabatic wall. There was good agreement between the conjugate heat transfer predictions of the overall cooling effectiveness with the experimental data.Copyright