Study of pollutant emissions and dynamics of non-premixed turbulent oxygen enriched flames from a swirl burner

Abstract

The paper investigates the dynamics of methane – oxygen enriched air turbulent non-premixed swirled flames and their emission characteristics. The burner configuration consists of two concentric tubes with a swirler placed in the annular part supplying the oxidant. Fuel injection is radial through holes in the central tube at the burner exit. This allows enhancing air-fuel mixing and eases flame stabilisation. Fuel is injected into the highest turbulence level regions where the local mixing is enhanced, and generating a shift from a non-premixed flame towards a partially premixed one. The idea of oxygen enrichment of the air is related to the augmentation of CO₂ concentration in the flue gases to improve the CO₂ capture efficiency by membranes. Stereo-Particle Image Velocimetry is used to analyse the velocity flow fields. The measurements are performed for oxygen concentrations ranging from 21% to 30%, with swirl numbers from 0.8 to 1.4 and global equivalence ratio from 0.8 to 1. Results show that combustion noticeably affects the swirling motion and that tangential velocities rapidly decrease along the axial axis. The recirculation mass flow ratio dramatically increases compared to non-reactive cases. Increasing global equivalence ratios increases the recirculation mass flow ratio contrary to oxygen enrichment effects. The exhaust gas compositions are measured using gas analysers. It is observed that NO_x emissions decrease when the global equivalence ratio increases. It suggests that CO₂ in the recirculated burned gases could be responsible for NO_x destruction at relatively high oxygen enrichment rates. Globally, the results show that, by combining enhanced mixing and moderate oxygen enrichment of the air, the developed and characterized new burner configuration has a good potential to be used to stabilize non-premixed fuel and oxidant streams as partially premixed low emission flames with an increased concentration of CO₂ in the flue gases.

Introduction

The use of gaseous fuels for energy generation, such as natural gas but also more and more synthetic gases from various gasification processes, is growing worldwide. Gaseous fuels emit less CO₂ compared to coal or fuel oil and the recent rush to shale gas in the US and the development of large scale coal gasification plants in China, will keep this resource alive for several decades [1]. In most gaseous fuel applications for industrial combustion, the diffusion or non-premixed combustion mode is preferred for obvious safety reasons [2]. This is all the more justified when hydrogen or hydrogenated fuels such as syngas, are used [3]. It is well known that the NO_x generation propensity of non premixed flames is higher compared to premixed flames [4]. This emission problem combined with the safety issue indicates the relevance of developing innovative burner configurations which rapidly convert a non-premixed flame to a premixed or at least-partially premixed one. Such a burner should ideally prevent any explosion risk by avoiding premixing the fuel with the oxidant before ignition and generate a premixed like flame after ignition. This paper discusses the characteristics of such a burner configuration which combines a swirling co-flow of the oxidant with a radial injection of the fuel [5], [6], [7].

Another issue today is the CO₂ capture from industrial flames. This constraint will become more stringent in the future and will be applied not only to very large power plants but also to medium scale industrial combustion systems. Post-combustion CO₂ capture is today a costly process when chemical capture solutions are applied [8]. Oxy-fuel combustion is an elegant solution but disadvantaged by the energetic penalty of oxygen generation. Post-combustion capture of CO₂ using physical processes such as membranes appear as a viable solution at least until low cost oxygen production technologies is available [9], [10]. Membrane capture systems of CO₂ require, however, CO₂ concentrations not smaller than 20% in flue gases [11]. This can be obtained by partial oxygen enrichment of the reactive mixture [12], [13]. One negative aspect of this approach is obviously its NO_x enhancing effect [14], [15]. Combining an innovative burner configuration allowing enhanced mixing with a moderate oxygen enrichment to enable CO₂ capture by membranes is the challenge of the present work.

In the following paragraphs the burner configuration is first described and characterized by stereo-PIV under non-reacting and reacting conditions. Instantaneous and averaged OH images are also obtained to help characterize the flame structure. Post flame emission measurements of CO₂, CO and NO_x are performed to discuss the merits of the developed burner configuration and the future research paths for its improvement.