George A. Richards

Issues for low-emission, fuel-flexible power systems

Modern power generation systems can produce clean, economical energy. Gas turbines, modern reciprocating engines and fuel cells may all play a role in new power production, both for electric power and mechanical drive applications. Compared to their counterparts of even a decade ago, new power systems have significantly reduced pollutant emissions. However, the careful balance between low emissions and operating performance often requires that system performance be optimized on a single fuel. Thus, for example, a gas turbine designed to produce low emissions on natural gas may not easily achieve the same emission goals on a different gaseous fuel. This paper reviews the various issues associated with changes in gaseous fuel composition for low-emission turbines, reciprocating engines and fuel cells.

Assessment of Rich-Burn, Quick-Mix, Lean-Burn Trapped Vortex Combustor for Stationary Gas Turbines

This paper describes the evaluation of an alternative combustion approach to achieve low emissions for a wide range of fuel types. This approach combines the potential advantages of a staged rich-burn, quick-mix, lean-burn (RQL) combustor with the revolutionary trapped vortex combustor (TVC) concept. Although RQL combustors have been proposed for low-Btu fuels, this paper considers the application of an RQL combustor for high-Btu natural gas applications. This paper will describe the RQL/TVC concept and experimental results conducted at 10 atm (1013 kPa or 147 psia) and an inlet-air temperature of 644 K (700°F). The results from a simple network reactor model using detailed kinetics are compared to the experimental observations. Neglecting mixing limitations, the simplified model suggests that NOx and CO performance below 10 parts per million could be achieved in an RQL approach. The CO levels predicted by the model are reasonably close to the experimental results over a wide range of operating conditions. The predicted NOx levels are reasonably close for some operating conditions; however, as the rich-stage equivalence ratio increases, the discrepancy between the experiment and the model increases. Mixing limitations are critical in any RQL combustor, and the mixing limitations for this RQL/TVC design are discussed.

Thermal Pulse Combustion

Abstract This paper describes theoretical and experimental observations of combustion oscillations produced in a continuously mixed, jet-stirred combustion system. This work is distinct from other investigations of pulse combustion, because it is shown both theoretically and experimentally that combustion oscillations can be produced with a steady supply of fuel and air, requiring no mechanical or aerodynamic valves. The theory is a direct extension of thermal theories of combustion in back-mixed reactors, extended to include the unsteady behavior of the combustor and tailpipe. Because of the demonstrated effect of heat transfer on the oscillations, the name thermal pulse combustion is chosen to describe these oscillations. Effects of friction in the combustor tailpipe, heat loss from the combustion zone, and flow rate, are investigated theoretically. Depending on operating parameters, oscillating combustion, steady flames, or blow-out are all predicted. The effects of finite mixing rate are investigated ...

SYSTEM ISSUES AND TRADEOFFS ASSOCIATED WITH SYNGAS PRODUCTION AND COMBUSTION

The purpose of this article is to provide an overview of the basic technology of coal gasification for the production of syngas and the utilization of that syngas in power generation. The common gasifier types, fixed/moving bed, fluidized bed, entrained flow, and transport, are described, and accompanying typical product syngas compositions are shown for different coal ranks. Substantial variation in product gas composition is observed with changes in gasifier and coal feed type. Fuel contaminants such as sulfur, nitrogen, ash, as well as heavy metals such as mercury, arsenic, and selenium, can be removed to protect the environment and downstream processes. A variety of methods for syngas utilization for power production are discussed, including both present (gas turbine and internal combustion engines) and future technologies, including oxy-fuel, chemical looping, fuel cells, and hybrids. Goals to improve system efficiencies, further reduce NOx emissions, and provide options for CO2 sequestration require advancements in many aspects of IGCC plants, including the combustion system. Areas for improvements in combustion technology that could minimize these tradeoffs between cost, complexity, and performance are discussed.

Chaos in thermal pulse combustion

An experimental thermal pulse combustor and a differential equation model of this device are shown to exhibit chaotic behavior under certain conditions. Chaos arises in the model by means of a progression of period-doubling bifurcations that occur when operating parameters such as combustor wall temperature or air/fuel flow are adjusted to push the system toward flameout. Bifurcation sequences have not yet been reproduced experimentally, but similarities are demonstrated between the dynamic features of pressure fluctuations in the model and experiment. Correlation dimension, Kolmogorov entropy, and projections of reconstructed attractors using chaotic time series analysis are demonstrated to be useful in classifying dynamical behavior of the experimental combustor and for comparison of test data to the model results. Ways to improve the model are suggested. (c) 1995 American Institute of Physics.

Combustion Oscillation Control by Cyclic Fuel Injection

A number of recent articles have demonstrated the use of active control to mitigate the effects of combustion instability in afterburner and dump combustor applications. In these applications, cyclic injection of small quantities of control fuel has been proposed to counteract the periodic heat release that contributes to undesired pressure oscillations. This same technique may also be useful to mitigate oscillations in gas turbine combustors, especially in test rig combustors characterized by acoustic modes that do not exist in the final engine configuration. To address this issue, the present paper reports on active control of a subscale, atmospheric pressure nozzle/combustor arrangement. The fuel is natural gas. Cyclic injection of 14% control fuel in a premix fuel nozzle is shown to reduce oscillating pressure amplitude by a factor of 0.30 (i.e., {approximately}10 dB) at 300 Hz. Measurement of the oscillating heat release is also reported.

A test device for premixed gas turbine combustion oscillations

This paper discusses the design and operation of a test combustor suitable for studying combustion oscillations caused by a commercial-scale gas turbine fuel nozzle. Aside from the need to be conducted at elevated pressures and temperatures, it is desirable for the experimental device to be flexible in its geometry so as to provide an acoustic environment representative of the commercial device. The combustor design, capabilities, and relevant instrumentation for such a device are presented, along with initial operating experience and preliminary data that suggests the importance of nozzle reference velocity and air temperature.

A study of techniques for reducing ash deposition in coal-fired gas turbines

Abstract Direct coal-fired gas turbines are potentially attractive alternatives to conventional steam cycle electric power generation because of their higher efficiencies. However, the high mineral matter content of coal creates problems with deposition, erosion, and corrosion of turbine components. Reported here is a study of methods to reduce the rate of formation of ash deposits, including a study of the effects of the addition of potential sulfur sorbents on downstream deposition problems. Methods studied involved modifying the combustion history of the coal particles in the combustor, cooling turbine components, and adding small amounts of aluminosilicate to the coal. The results show that the proper combination of these techniques can produce a two orders of magnitude reduction in the rate of ash deposit formation.

Ash deposition at coal-fired gas turbine conditions; Surface and combustion temperature effects

In this paper a study of ash deposition from a cleaned bituminous and conventional bituminous coal is presented. An electrically heated drop tube furnace is used to burn the coal and provide deposition conditions representative of proposed coal-fired gas turbines. Variations in the combustion temperature and deposit surface temperature demonstrate that surface cooling may significantly reduce ash deposition, or may provide little benefit, depending on the combustion conditions. Lower temperature combustion produced larger ash particles, with a greater fraction of ash adhering to the deposition test surface. Although the sticking coefficient was higher at the lower combustion temperature, the deposits were readily removed. A modest numerical simulation suggests that the smallest ash particles can experience significant boundary layer cooling and may account for the reduction in sticking observed at some conditions.

Characterization of Oscillations During Premix Gas Turbine Combustion

The use of premix combustion in stationary gas turbines can produce very low levels of NOx emissions. This benefit is widely recognized, but turbine developers routinely encounter problems with combustion oscillations during the testing of new pre mix combustors. Because of the associated pressure fluctuations, combustion oscillations must be eliminated in a final combustor design. Eliminating these oscillations is often time-consuming and costly because there is no single approach to solve an oscillation problem.Previous investigations of combustion stability have focused on rocket applications, industrial furnaces, and some aeroengine gas turbines. Comparatively little published data is available for premised combustion at conditions typical of an industrial gas turbine.In this paper, we report experimental observations of oscillations produced by a fuel nozzle typical of industrial gas turbines. Tests are conducted in a specially designed combustor, capable of providing the acoustic feedback needed to study oscillations. Tests results are presented for pressures up to 10 atmospheres, and with inlet air temperatures to 588 K (600 F) burning natural gas fuel.Based on theoretical considerations, it is expected that oscillations can be characterized by a nozzle reference velocity, with operating pressure playing a smaller role. This expectation is compared to observed data, showing both the benefits and limitations of characterizing the combustor oscillating behavior in terms of a reference velocity rather than other engine operating parameters. This approach to characterizing oscillations is then used to evaluate how geometric changes to the fuel nozzle will affect the boundary between stable and oscillating combustion.© 1997 ASME