Richard Marsh

Evaluation of Methods for Measuring Particulate Matter Emissions from Gas Turbines

The project SAMPLE evaluated methods for measuring particle properties in the exhaust of aircraft engines with respect to the development of standardized operation procedures for particulate matter measurement in aviation industry. Filter-based off-line mass methods included gravimetry and chemical analysis of carbonaceous species by combustion methods. Online mass methods were based on light absorption measurement or used size distribution measurements obtained from an electrical mobility analyzer approach. Number concentrations were determined using different condensation particle counters (CPC). Total mass from filter-based methods balanced gravimetric mass within 8% error. Carbonaceous matter accounted for 70% of gravimetric mass while the remaining 30% were attributed to hydrated sulfate and noncarbonaceous organic matter fractions. Online methods were closely correlated over the entire range of emission levels studied in the tests. Elemental carbon from combustion methods and black carbon from optical methods deviated by maximum 5% with respect to mass for low to medium emission levels, whereas for high emission levels a systematic deviation between online methods and filter based methods was found which is attributed to sampling effects. CPC based instruments proved highly reproducible for number concentration measurements with a maximum interinstrument standard deviation of 7.5%.

Augmenting the Structures in a Swirling Flame via Diffusive Injection

Small scale experimentation using particle image velocimetry investigated the effect of the diffusive injection of methane, air, and carbon dioxide on the coherent structures in a swirling flame. The interaction between the high momentum flow region (HMFR) and central recirculation zone (CRZ) of the flame is a potential cause of combustion induced vortex breakdown (CIVB) and occurs when the HMFR squeezes the CRZ, resulting in upstream propagation. The diffusive introduction of methane or carbon dioxide through a central injector increased the size and velocity of the CRZ relative to the HMFR whilst maintaining flame stability, reducing the likelihood of CIVB occurring. The diffusive injection of air had an opposing effect, reducing the size and velocity of the CRZ prior to eradicating it completely. This would also prevent combustion induced vortex breakdown CIVB occurring as a CRZ is fundamental to the process; however, without recirculation it would create an inherently unstable flame.

Thermal degradation of polyethylene film materials due to successive recycling

Abstract Mechanical recycling of plastic film involves subjecting plastic materials to a series of heat cycles that can potentially degrade the material, causing brittleness and increased melt viscosity. Plastic film recycling in the UK is in its infancy, in need of an increased understanding of how the physical properties of polymeric materials change before and during the process. Reliable data are required to estimate the behaviour of such film products when recycled. Measurements were made as to the changes in physical properties of four different varieties of polyethylene (PE) film products when subjected to a series of successive simulated heat cycles and evaluated after each step. Results showed that although changes in tensile properties were fairly small, changes in processing properties such as melt-flow index for highly branched or low-density PE are substantial and could be a concern during recycling operations.

Methane-Oxygen Flame Stability in a Generic Premixed Gas Turbine Swirl Combustor at Varying Thermal Power and Pressure

At low thermal power (<5 kW) conditions, nitrogen and carbon dioxide were added as diluents to a premix of methane-oxygen in an atmospheric generic swirl burner. Results indicate that CO2-diluted oxy-methane flames have a wider stability range than N2-diluted flames in terms of overall oxygen concentration in the premix. Bulk flow Reynolds number, augmented by varying the size of the burner exit nozzle, was also found to increase the stability limits of flames diluted with both CO2 and N2, as the increased flow velocity offsets the higher burning velocity of the oxyfuel mixture. A combination of differing transport properties between diluents and the resulting flame chemistry produces a change in the structure of the premixed oxyfuel swirl flame, shown by combustion PIV to affect the observed lean and rich stability limits. Utilising the results at low thermal power conditions, enhanced-oxygen combustion of a methane-air flame was investigated in a pressurized generic swirl burner operating at higher thermal power (<50 kW) conditions and pressures up to 3 bar absolute. Over a range of increasing thermal powers, it is seen that a relatively small amount of pure oxygen addition can shift the equivalence ratio at which the lean stability limit or rich stability limit are reached compared with the same phenomenon observed for a methane-air flame. Pressurised operation with CO2 dilution up to 15.5 mol% was validated through stability limit and emissions gas analysis, giving further support to the use of exhaust gas recirculation in premixed swirl-stabilized burners for oxyfuel combustion.

The use of CO2 to improve stability and emissions of IGCC combustors

The use of gas for power generation is likely to increase in the medium term. Also, the introduction of new fuels will ensure a higher generation with lower emissions under continuous operation. These scenarios lead to the conclusion that there will be a considerably more diverse range of fuel supply. However, the use of these new fuels contrasts with recent experiences of global operators who report increasing emissions and difficult combustion dynamics with even moderate variations in their fuel characteristics. Clearly there are significant challenges for fuel flexible gas turbines, particularly emission control, combustor dynamics and flame stability. Trials using a power derivative gas turbine combustor and a high hydrogen content fuel produced unusual flashback events, in that flashback was induced by either leaning of the fuel mixture by the increase of combustion air, or by a change in composition through the reduction of methane pilot fuel. The introduction of CO2 through the combustors pilot injector prevented flashback from occurring under these circumstances. The resulting reduction of temperature in the combustion zone, indicated by lower burner tip temperatures causes a reduction in the emissions of nitrous oxides, whilst there is minimal effect on the effective turbine inlet temperature, only a 2.3% reduction. Investigations using a ‘generic’, radial swirl burner and stereo PIV demonstrated how the flashback depended on a combination of flow structure augmentation and changes in mixture burning rate. The injection of methane or CO2 had differing effect on these parameters of the combustion zone, but both produced combinations that facilitated stability.

Preliminary Results from a High Pressure Optical gas Turbine Combustor Model with 3D Viewing Capability

This paper will describe the evolution of a high pressure optical gas turbine rig with viewing capability from 3 dimensions. Commisioning work is described followed by an introduction to a generic swirl burner, scaled up by 40 % from a well characterised atmospheric pressure unit. One of the major features of the system is the ability to carry out full 3D studies via the use of an extra optical window which allows flow and other parameter characterization in the rθ direction. Non intrusive laser diagnostics have been installed to use this, with the analysis shown with dynamic pressure transducers to give thermoacoustic relevance to the heat release detection capability.

Variation in laminar burning velocity and Markstein length with water addition for industrially produced syngases

An outwardly propagating spherical flame has been used to characterise the influence of water addition on the combustion of variable steelworks gas compositions. Attention was given to the ratio of hydrogen and carbon monoxide within blast furnace gas, and the catalysing influence of water addition on the preponderant reaction kinetics. A nonlinear extrapolative technique was used to obtain values of laminar burning velocity and Markstein length for atmospheric combustion with air and change in equivalence ratio. Four disparate blast furnace gas mixtures were tested with increasing volumetric proportions of hydrogen in the range of one to seven percent, displacing other constituent fractions. A non-monotonic influence was observed, with propagation accelerated for compositions comprising smaller amounts of hydrogen, and the cooling impact of water addition shown to slow faster burning flames. Water addition was also shown to increase the effects of flame stretch on observed propagation rates, and the contrasting influences resulting from vapour fraction are discussed with respect to practical combustion instability, in addition to alternative synthesised fuels. Numerically modelled results were generated using the PREMIX coded CHEMKIN-PRO, and the performance of specified chemical reaction mechanisms evaluated in relation to the obtained experimental data.

Laminar Burning Velocity and Markstein Length Characterisation of Compositionally Dynamic Blast Furnace Gas

Blast Furnace Gas is a poor quality process gas comprising proportions of CO, H2, CO2, and N2, with a low energy density typically in the order of 3 MJ·kg−1. Produced in large quantities as a by-product of blast furnace iron making, it is one of the process gases indigenous to integrated steelworks worldwide. The inherently dynamic nature of furnace operation causes compositional variation and therefore leads to fluctuation in the fuel characteristics, often dissuading engineers from fully utilising the gas in increasingly complex and efficient technologies such as gas turbines. Characterisation studies were undertaken in a new constant volume bomb to determine the sensitivity to change in laminar burning velocity and Markstein length experienced as a result of increasing the volumetric H2 fraction in the range of 1–7%. Experiments were performed by measuring outwardly propagating spherical flame evolution, recorded using a Schlieren flame visualisation technique for a range of equivalence ratios, and processed using nonlinear data analysis. The relative performance of the experimental technique was benchmarked against other works using well-investigated CH4 and yielded results in good agreement with published values. Peak laminar burning velocity was shown to increase by a factor of approximately 3.5 over the tested range, with H2 concentration and equivalence ratio shown to greatly influence the effect of flame stretch. Comparisons of results were also made with values obtained from different reaction mechanisms employed using the PREMIX code developed by Sandia National Laboratories.

Temperature measurement of gas turbine swirling flames using tomographic imaging techniques

This paper presents the 3-D (three-dimensional) temperature measurement of swirling flames of a well-characterized tangential swirl burner using a RGB (red, green and blue) CMOS (Complementary metal-oxide-semiconductor) camera associated with four flexible imaging fiber bundles for flame image acquisition. Optical tomographic algorithms were used to reconstruct the 3-D model of grey-level intensity of the flame and the two-color pyrometric technique was applied for computing the flame temperature based on the reconstructed 3-D model. Three R-type thermocouples were also employed to measure the flame temperature which was then used as a reference for validating the temperature derived from the flame images. Experimental results obtained show that the proposed technique is capable of determining flame temperature profiles, and consequently can be an effective means of characterizing the 3-D swirling flame behaviors, including stability limits such as flame blow-off/flashback, thus reducing the event probability by changing inlet conditions.

Experimental investigation of the effects of central fuel injectors on premixed swirling flames

The demand for alternative fuels has increased significantly during the previous decades in order to reduce pollutants and increase the amount of energy that can be generated from non-fossil fuels. However, the use of new fuels faces many issues especially the problem of stability of operation which sometimes can cause severe damages to the system hardware. Thus the development of flexible combustion systems for gas turbines becomes urgent in order to achieve high reliability with these new sources of energy. Swirl stabilized combustion is the most widely spread deployed technology used to stabilize and control combustion in gas turbines and numerous other systems. However, the interaction of the swirling flows with the burner geometries is very complex and it has been proved that any change in the burner geometry can affect the flow field inside the combustion chamber, close to the burner mouth and downstream the combustion zone. Most burners are generally provided with a diffusive injector that centrally delivers well-known fuels allowing the stabilization of the system previous to entirely premixed conditions. Moreover, the injector anchors the central recirculation zone formed downstream of the nozzle. However, the use of injectors can also affect the stability limits of the system, especially the propagation of flashback through changes of shape of the shear layer since other structures such as the Combustion Induced Vortex Breakdown are suppressed due to the presence of this central body. However, the characterization of the flow and its impacts on the propagation of these and other flashback structures using different injectors has been briefly documented. Thus, this paper presents a series of experiments using a well-characterized tangential swirl burner to determine the impact of different central injector geometries on the flow field characteristics which directly affect the flow stagnation point downstream of the burner mouth and consequently the propagation of the Combustion Induced Vortex Breakdown. Results show how the use of various injectors and swirl numbers can impact on the flashback limits with a minimum outside diameter before which the Combustion Induced Vortex Breakdown is altered.