Effects of Hydrogen Enrichment on the Reattachment and Hysteresis of Lifted Methane Flames

The purpose of this study is to observe the effects of hydrogen enrichment on the stability of lifted, partially premixed, methane flames. Due to the relatively large burning velocity of hydrogen-air flames when compared to that of typical hydrocarbon-air flames, hydrogen enriched hydrocarbon flames are able to create stable lifted flames at higher velocities. In order to assess the impact of hydrogen enrichment, a selection of studies in lifted and attached flames were initiated. Experiments were performed that focused on the amount of hydrogen needed to reattach a stable, lifted methane jet flame above the nozzle. Although high fuel velocities strain the flame and cause it to stabilize away from the nozzle, the high burning velocity of hydrogen is clearly a dominant factor, where as the lifted position of the flame increased, the amount of hydrogen needed to reattach the flame increased at the same rate. In addition, it was observed that as the amount of hydrogen in the central jet increased, the change in flame liftoff height increased and hysteresis became more pronounced. It was found that the hysteresis regime, where the flame could either be stabilized at the nozzle or in air, shifted considerably due to the presence of a small amount of hydrogen in the fuel stream. The effects of the hydrogen enrichment, however small the amount of hydrogen compared to the overall jet velocity, was the major factor in the flame stabilization, even showing discernible effects on the flame structure.

Assessment of Stabilization Mechanisms of Confined, Turbulent, Lifted Jet Flames: Effects of Ambient Coflow

The aim of this investigation is to determine the effects of confinement on the stabilization of turbulent, lifted methane (CH4) jet flames. A confinement cylinder (stainless steel) separates the coflow from the ambient air and restricts excess room air from being entrained into the combustion chamber, and thus produces varying stabilization patterns. The experiments were executed using fully confined, semi-confined, and unconfined conditions, as well as by varying fuel flow rate and coflow velocity (ambient air flowing in the same direction as the fuel jet). Methane flames experience liftoff and blowout at well-known conditions for unconfined jets, however, it was determined that with semi-confined conditions the flame does not experience blowout. Instead of the conventional unconfined stabilization patterns, an intense, intermittent behavior of the flame was observed. This sporadic behavior of the flame, while under semi-confinement, was determined to be a result from the restricted oxidizer access as well as the asymmetrical boundary layer that forms due to the viewing window. While under full confinement the flame behaved in a similar method as while under no confinement (full ambient air access). The stable nature of the flame while fully confined lacked the expected change in leading edge fluctuations that normally occur in turbulent jet flames. These behaviors address the combustion chemistry (lack of oxygen), turbulent mixing, and heat release that combine to produce the observed phenomena.

Numerical Simulation of Turbulent Combustion Flows for Coaxial Jet Cluster Burner

We have developed a burner for the gas turbine combustor, which was high efficiency and low environmental load. This burner is named the “coaxial jet cluster burner” and, as the name indicates, it has multiple fuel nozzles and holes in a coaxial arrangement. To form lean premixed combustion, this burner mixes fuel and air in the multiple holes rapidly. The burner can change the combustion form between premixed and non-premixed combustion by controlling the mixing. However, the combustion field coexisting with premixed and non-premixed combustion is complicated. The phenomena that occur in the combustion field should be understood in detail. Therefore, we have developed the hybrid turbulent combustion (HTC) model to calculate the form in which non-premixed flame coexists with premixed flame. Turbulent flow has been simulated using a large eddy simulation (LES) with a dynamic sub grid scale (SGS) model coupled with the HTC model. These models were programmed to a simulation tool based on the OpenFOAM library. However, there were unclear points about their applicability to an actual machine evaluation and the predictive precision of CO concentration which affects burner performance. In this study, we validate the HTC model by comparing its results with measured gas temperature and gas concentration distributions obtained with a coaxial jet cluster burner test rig under atmospheric pressure. In addition, we analyze the CO generation mechanism for the lean premixed combustion in the burner.

Cyclic Operation of Supercritical High Pressure Feedwater Heater Study

As more renewable energy sources come on line with the inherent inconsistency of load dispatch feedwater heaters become subject to more frequent and rapid cyclic operation. In a recent project, American Exchanger Services (AM-EX) was asked to gather and analyze operating information on a high pressure feedwater heater during daily rapid load changes. This particular supercritical coal plant was designed to operate in flexible load environments, thus acquiring data during the summer months was optimal. The heater was run from rest to full power while temperature data was acquired. All data from the study and supporting plant information was used to generate models for preparing maintenance projections, informing future designs, and repair recommendations. The primary component of focus is the desuperheating zone exhaust where tube failure tends to be greatest caused by wet wall conditions. The result of the analysis was less conclusive than was anticipated. Actual performance of the heaters is a key issue and there were specific indications that the heaters were not performing to specifications. A more detailed thermal performance analysis using the ASME PTC12.1 should be considered to accurately determine the extent to which the heaters are meeting design performance.

Numerical Study of Methane and Air Combustion Inside Heat-Recirculating Combustors

A numerical model on methane/air combustion inside a small Swiss-roll combustor was set up to investigate the flame position of small-scale combustion. The simulation results show that the combustion flame could be maintained in the central area of the combustor only when the speed and equivalence ratio are all within a narrow and specific range. For high inlet velocity, the combustion could be sustained stably even with a very lean fuel and the flame always stayed at the first corner of reactant channel because of the strong convection heat transfer and preheating. For low inlet velocity, small amounts of fuel could combust stably in the central area of the combustor, because heat was appropriately transferred from the gas to the inlet mixture. Whereas, for the low premixed gas flow, only in certain conditions (Φ = 0.8 ～ 1.2 when ν0 = 1.0m/s, Φ = 1.0 when ν0 = 0.5m/s) the small-scale combustion could be maintained.

A Dry Low-NOx Gas-Turbine Combustor With Multiple-Injection Burners for Hydrogen-Rich Syngas Fuel: Testing and Evaluation of its Performance in an IGCC Pilot Plant

Success of oxygen-blown integrated coal gasification combined cycle (IGCC) technology requires gas turbines capable of achieving dry low nitrogen oxides (NOx) combustion of hydrogen-rich syngas for low emissions and high plant efficiency. The authors have been developing a “multiple-injection burner” to achieve dry low-NOx combustion of such hydrogen-rich fuels using surrogate fuel composed of hydrogen, nitrogen, and methane. The purpose of this paper is to report test results of a multi-can combustor equipped with multiple-injection burners for a practical syngas fuel in an IGCC pilot plant and to evaluate its performance. The syngas fuel consisted of hydrogen, nitrogen, and carbon monoxide up to approximately half of its volume. In the test, the combustor achieved stable and reliable operation from ignition through partial load to the maximum load, and achieved NOx emissions of 15.1 ppm (at 15% oxygen) at the maximum load. These findings demonstrated that the combustor achieves dry low-NOx combustion of the syngas fuel in the IGCC pilot plant.

Assessment of Life of Pressure Vessels and Pipes in Crude Oil and Gas Industries

Fitness-for-service (FFS) assessments are quantitative engineering evaluations which are required to be preformed periodically in accordance with the published codes and standards to demonstrate the structural integrity of in-service components. This report summarizes the results of nondestructive in-service-inspection (ISI) of pressurized components conducted for condition assessment of the Dakhani Gas Processing Plant of Oil and Gas Development Corporation Ltd. (OGDCL) for the first time since its commissioning in December,1989. The non-destructive evaluation of the plant was required because of concerns for occurrence of sulphide-stress-cracking. Hydrogen embrittlement, hydrogen-including-cracking, weight-loss-corrosion, sulphur-stress-corrosion due to determental service conditions at Dakhani having low PH, High H2S, high chlorides and pressure of CO2. The results have shown that microstructural changes associated with first and second stage of hydrogen attack have occurred in almost all of the pipe joints and pressure vessels. Hardness of some vessels has even exceeded the NACE limit of 220 HB. Effect of second stage of hydrogen attack are dominant in pipe joints, resulting in loss of hardness and strength because of decarburization. The results based on ultrasonic attenuation monitoring also indicate degradation of components. Random rounded indications have also been observed in some pipe joints during X-Ray radiographic testing that could serve as sites for failure initiation. The corrosion-under-insulation is observed for joints of piping spreading over a significant length. Localized corrosion and pitting is also observed in some locations of pressure vessels and piping. Ultrasonic thickness gauging has shown a significant variation in thickness for dish end and shell of some pressure vessels as well as for various joints of piping. In absence of periodic ISI data for the plant and keeping in view the results of non-destructive evaluation summarized above, the end-of-life (EOL) assessment of pressure vessels and piping is not possible and operation of the plant should be continued with a degree of caution. Any estimate of safe life assessment of the plant made at this stage would require revision on the basis of observed level of degradation through essential periodic in-service monitoring. In order to cope with the situation, it is recommended that monitoring of further degradation of microstructure and hardness along with flaw growth should be carried out after a period of 8x103 hours. Necessary remedial measures for rectification of flaws are requested. Non-destructive strain gauging is recommended to estimate data for safe life assessment of pressure vessels. Thermographic scanning of on-line in-service insulated pipelines is proposed for monitoring corrosion-under-insulation during plant operation.

Scale Model Testing Towards Primary Air Optimization of a CFB Boiler

Primary air flow is supplied to the wind box of a 300 MW CFB boiler by means of three primary air ducts connected to a common plenum. A flow rate measurement is performed in each duct using an Annubar flow meter. Due to the tight configuration of the piping and the associated turbulence, the measured flows in the three ducts were not consistent, resulting in improper air flow distribution in the boiler. A primary air duct flow study was performed in an effort to improve measurement accuracy, which will lead to improved combustion efficiency and low load cycling by allowing precision air flow control in the furnace. The objective of this project was to employ a 1/10th scale model test to determine the best method to improve the flow upstream of each Annubar flow meter. Tests were conducted with room temperature air at several flow rates generated by a centrifugal fan. Reynolds number independence was achieved in the tests. Appropriate dimensionless Grashof and Prandtl number scaling was used to extrapolate the scale model tests to the high temperature conditions prevalent in the unit. Flow visualization and measurements using a five-hole directional velocity probe were performed. It was determined that mounting 50% open area screens in the primary air duct of the scale model had the effect of significantly reducing both yaw and pitch angles across the Annubar measurement plane. A complimentary benefit was that the velocity profiles in the flow direction were notably flattened by the addition of the screens. Furthermore the performance of the Annubar flow meters could be improved if the measurement plane was moved further upstream of the opposed blade flow control damper.

Closed Cooling Water Heat Exchanger Design for Higher Tube Inlet Water Temperature

When the maximum temperature of cooling water slowly increases with temperature changes and shifting climate patterns, smaller LMTD’s (log mean temperature differences) for the CCW’s to meet the same performance heat rejection. Making the issue more critical is that the peak cooling water temperatures will usually occur at the same time as peak summer load demand. A smaller LMTD means a larger heat exchanger and more effective tubing surface area. More surface, means more tubing or smaller diameter tubing. If the original LMTD was 12 °F, a 1 degree change may mean an increase of 9%. To maintain the same nozzle locations on a replacement exchanger means a smaller tube outside diameter and/or a larger shell. Such increases are necessary for the high summer load conditions with the highest inlet water temperatures. At lower water temperatures, the amount of excess thermal capability can become a performance and corrosion issue as the water flows are modulated to meet temperatures. To help reduce these problems, a design which allows operation with reduced surface at low temperatures is appropriate. The temperature approach (Cooling Water Out – Service Water In) based on the higher inlet cooling water temperature can be significantly smaller than when the CCW was originally designed. This paper will address a design configuration that will work with both higher summer temperature cooling water with the flexibility of using less water for cooler winter operation. The overall affect is less pumping power during colder months, more consistent tube velocities which will help with heat transfer, and minimization of sediment settling in the tubes due to lower velocities.

An Engineer’s Guide to Eddy Current Testing

This paper applies to individuals charged with maintaining the reliability of shell and tube heat exchangers. These persons typically specify and/or retain the services of others to examine heat exchangers with nondestructive test methods, such as eddy current and are responsible for submitting run-repair-replace recommendations to management. Electromagnetic Testing (ET) uses the electromagnetic characteristics of components made of conductive materials to determine their condition. Eddy Current Testing (ECT), an electromagnetic method that utilizes induced electrical currents, is usually used to examine non-ferromagnetic materials. ECT’s high rate of examination, relatively good accuracy with thin wall components, repeatability and volumetric measurement make it an ideal method for examining nonmagnetic heat exchanger tubes. This paper will provide a brief description of the method, concentrating on ECT because most power generation industry heat exchanger tubing is non-ferromagnetic in nature. This paper will also address the following: • Training and Certification of Technicians. • ET signal analysis, an exacting science? • ASME Section V, Appendix II vs. Appendix VIII for in-situ ECT of all heat exchanger tubing. • Signal analysis variables and limitations. • A need to know the potential degradation mechanisms. • Condition assessment vs. eddy current testing.