Comparison and parametric study of flameless oxidation in a gas turbine using two kinetics mechanisms

INTRODUCTION

Environmental concerns and limited resources of fuels have been the major constraints in designing combustion systems. These constraints have triggered researchers and manufacturers of combustion systems to develop low polluting and fuel-efficient combustion systems. The major pollutants produced by combustion are unburned and partially burned hydrocarbons, nitrogen oxides or [NO.sub.x] (NO, [NO.sub.2]), carbon monoxide (CO) and sulphur oxides ([SO.sub.2] and [SO.sub.3]). Nitrogen oxide is one of the most toxic pollutants in the atmosphere and is well known as a destroyer of stratospheric ozone and a precursor of acid rain. Different process alternatives have been proposed to reduce the emissions from combustion devices. These include, for example, exhaust gas recirculation, air staging, re-burning and low [NO.sub.x] burners. In general, these methods try to reduce the residence time in high temperature regions, or to avoid high oxygen concentration in such regions. In this way, the formation of thermal NO is largely suppressed, since this mechanism is highly dependent on the temperature and requires temperatures above 1800 K to produce a significant amount of NO.

Another key issue in the design and operation of combustion equipment is the combustion efficiency. A well known method to improve combustion efficiency is to use the exhaust gases to preheat the combustion air by means of a heat exchanger. However, this method generally yields an increase of the flame temperature, resulting in an increase of thermal NO formation and [NO.sub.x] emissions.

In the last few years significant efforts have been made to retain the benefit of better combustion efficiency due to air-preheating without the adverse effect of higher [NO.sub.x] emissions caused by higher flame temperatures. As a result of these efforts, it was found that strong exhaust gas recirculation combined with air preheating generates relatively low flame temperatures. The combustion air is diluted with a large amount of re-circulated exhaust gases and so the mass fraction of oxygen in the reaction zone is much lower than in the case of undiluted atmospheric air. Therefore, the stoeichiometric flame temperature is also lower. In addition, the temperature fluctuations are small and there is no luminosity or sound emission from the flame. This combustion regime was referred to as Flameless Oxidation or FLOX[TM] by Wunning and Wunning (1) and was reviewed by Katsuki and Hasegawa (2), who emphasized the research carried out in Japan.

Although the technology of fuel combustion with highly preheated air has substantially advanced over the last decade or so, there has been relatively little work undertaken by the fundamental combustion community to support the development. The work on fundamentals of the process was initiated probably by R. Tanaka (3) sometime at the beginning of the nineties. Soon afterwards Gupta et al. (4), (5) undertook research on the effect of combustion air temperature and oxygen concentration on flame colour, visibility and thermal emission spectra. Propane was used as the fuel. The researchers observed a substantial increase in radiation intensity at wavelengths corresponding to [C.sub.2] radicals emission with the preheated air temperature under low oxygen concentration conditions. Mochida and Hasegawa (6) have developed a flame visualization technique based on the luminescence intensity ratio of [C.sub.2] and CH radicals. Blasiak et al. (7) built an experimental facility for studying fuel jets immersing into a cross-flowing high temperature air stream to enhance mixing and attain flameless conditions. The IFRF carried out semi-industrial scale experiments (Verlaan et al. (8) and Weber et al. (9)) that identified the principal characteristics of the flameless combustion process. The furnace was operated almost like a well-stirred reactor. The measured radiative heat fluxes at the furnace walls were very high and uniform. Uniformity and homogenization of flameless combustion of methane and propane with highly preheated air (1000[degrees]C) have been studied by Ishiguro et al. (10). They obtained images of OH, CH and [C.sub.2] emissions for a number of experimental conditions that differed in the air preheat level and oxygen content of the air. The investigators concluded that the increase in air temperature resulted in a decrease of flame temperature gradients (homogenization of reaction zone) and low flame fluctuations. Plessing et al. (11) used laser-induced predissociative fluorescence and Rayleigh thermometry to examine flameless oxidation at laboratory scale. They observed that the flameless oxidation takes place in the well-stirred reactor regime. The OH concentration in the combustion zones of flameless oxidation is lower than in no-preheated undiluted turbulent premixed flames. De Joannon et al. (12) have examined the applicability of the existing chemical reaction schemes for combustion of hydrocarbons to high temperature air combustion conditions. Cavaliere and De Joannon (13) have argued that flameless oxidation can be described as a two staged combustion in which the first part is in rich conditions with plenty of inert gases.