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.

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.

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.

One Dimensional Model for Rotating Channels in the Turbine System: Part 2 — Formulation and Validation for Film Condensation

This paper provides an extension of the previously described [1] formulation of a one-dimensional model for steady, compressible flow inside a channel, to the steam turbine application. The major challenge faced in the network simulation of the steam turbine secondary system is the prediction of the condensation that occurs during the engine start-up on the cold parts that are below the saturation temperature. Neglecting condensation effects may result in large errors in the engine temperatures since they are calculated based on the boundary conditions (heat transfer coefficient and bulk temperature) which depend on the solution of the network analysis. This paper provides a detailed formulation of a one-dimensional model for steady, compressible flow inside a channel which is based on the solution of two equations for a coupled system of mass, momentum and energy equations with wall condensation. The model also accounts for channel area variation, inclination with respect to the engine axis, rotation, wall friction and external heating. The formulation was first validated against existing 1D correlation for an idealized case. The wall condensation is modeled using the best-suited film condensation models for pressure and heat transfer coefficient available in the literature and has been validated against the experimental data with satisfactory predictions.

Parallel Flow Boiler Designs to Minimise Erosion and Corrosion From Dust Loaded Flue Gases

Improving power plant performance, availability and operational costs is crucial to remain competitive in today’s competitive energy market. The boiler is a key component to achieve these objectives, particularly so when using challenging fuels, such as municipal solid waste or exhaust gases with high dust contents. This paper describes an innovative boiler design that has been used for the first time in an Energy from Waste plant in Bamberg, Germany. The new boiler design disregards the traditional heating surface arrangement and instead uses tube bundles arranged in parallel to the gas flow, which provides several advantages, such as reduced fouling. The paper describes the Bamberg project (boiler design and project highlights) and first operational results after 30,500h of operation. Additionally, the paper investigates further options to reduce fouling through the use of dimpled tubes, especially the ip tube® technology. The technology is presented as well as first test results of such tubes in the Energy from Waste plant Rosenheim, Germany. The paper concludes with further applications for the parallel flow boiler design, such as cement kilns, to outline future markets.

Effect of Toluene Addition on Hydrogen Sulfide Combustion Under Claus Condition

Experimental results on the effect of different amounts of toluene addition to H2S gas stream are presented. Three toluene concentrations of 0.5%, 1% and 5% in H2S are presented and compared with the baseline case of 100% H2S/air combustion. Temperature data showed that addition of toluene to H2S gas stream increases the flame temperature because of large heating value associated with toluene. Addition of toluene resulted in the production of H2, which increased with increase in the amounts of toluene addition. Furthermore, increased addition of toluene concentration increased the asymptotic value of hydrogen sulfide due to oxidation competition between the formed H2 and H2S. The results also showed that the presence of CO triggers the formation of COS with toluene addition due to reaction of CO with SO2. The results revealed that SO2 mole fraction increased to a maximum value then decayed with distance along the reactor. Addition of toluene increased the rate of SO2 decay. These results have direct impact on sulfur capture in Claus reactor performance for sulfur capture.

Environmentally-Benign Conversion of Biomass Residues to Electricity

As petroleum resources are finite, it is imperative to use them wisely in energy conversion applications and look for alternative options as an energy source. Biomass is one of the renewable energy sources that can be used to partially replace fossil fuels. Biomass-based fuels can be produced domestically and may thus reduce dependency on fuel imports. Due to their abundant supply, and given that to an appreciable extent they are considered to be carbon-neutral, their use for power generation is of technological interest. However, whereas biomasses can be directly burned in furnaces, such a conventional direct combustion technique is ill-controlled and typically produces considerable amounts of health-hazardous airborne compounds [1,2]. Thus, an alternative technology is described herein to further address our increasing energy needs and, at the same time, utilize our biomass streams in an environmentally-benign manner. More specifically, a multi-step process/device is outlined to accept biomass, of various types and shapes, and generate an easily-identifiable form of energy as a final product. To achieve low emissions of products of incomplete combustion, the biomass is gasified pyrolyticaly, mixed with air, ignited and, finally, burned in nominally premixed low-emission flames. Combustion is thus indirect, since the biomass is not directly burned, instead its gaseous pyrolyzates are burned upon mixing with air. Thereby, combustion is well-controlled and can be complete. A demonstration device has been constructed to convert the internal energy of plastics into clean thermal energy and, eventually to electricity.

Some Aspects of Steam Turbine Valves: Materials, Operations and Maintenance

Steam turbine valves are the most essential components of modern steam turbines from an operation, performance, reliability and safety aspects of a modern power plant. Current designs are pushing the operational envelope and it is not uncommon for large ultracritical plants to run on pressures exceeding 4500 psi and 1200 °F. These conditions are not only challenging for materials of construction for turbines and boilers but also for main steam turbine valves. The tendency of materials to oxidize at these temperatures is all too common causing problems for valve heads, stems, discs, bushings and seats. OEMs around the world are pushing to develop valve components with 9–12% Cr martensitic steels and nickel based alloys which offer better creep strength at elevated temperatures. For existing power plants at temperatures of a 1000 to 1050 °F range there is a push to retrofit valve components with Incolloy 901 type, Inconel 718 and Stellite alloys. Scale build up in traditional alloys happens too quickly for the usual two year maintenance cycle. The application of better alloys for steam turbine valves makes it possible to increase the maintenance cycle from two to four or even six years, while increasing the operational reliability of the valve. Elimination of main steam valve failures removes risks of turbine overspeed events and increases plant availability. Solid particle erosion is not forgiving on valve parts such as stems, discs and valve seats and over a period of time, excessive wear causes the valve to be rendered unsafe to continued service. Nitrided materials and chrome-carbide-coated materials are much harder than the stem base material; and to slow down wear, a nitriding process is used to develop a thin, hard, wear-resistant surface. Some of the material often used for Stellite liners are Nitralloy 135M, 410 SS, 422 SS Nitrided, Incolloy 901 Nitrided, 347 SS, 13Cr-13Ni-10Co-3Nb-2.5W-2Mo. Different OEMs use a variety of alloys for valve seats, discs and stems. Antigalling characteristics are particularly favorable. Valve casings are cast materials and usually specifications include the ASTM A217 and ASTM A356. The ASTM A217 cast steels are typically, 1.25Cr-0.5Mo Grade WC6 and the 2.25Cr-1Mo Grade WC9 materials. Some of the problems experienced with steam turbine valves, are sticking to the valve seat requiring excessive pull-out force, wear of the seat surface, valves not closing properly due to oxidation build up, Stellite weld cracking, cutting or gouging due to solid particle erosion. The material presented in this paper is of interest to fossil power plant personnel experiencing challenges on valve performance and maintenance. The paper looks at all aspects of steam turbine valves as far as current trends in valve material, operation and maintenance and lastly, looks at recent occurrences of valve failures leading to steam turbine overspeed catastrophic failures around the world.