| OCR Text |
Show ABSTRACT A novel liquid-fuel fired oxygen burner was developed to obtain homogeneous heat-transfer rates combined with low NOx emissions. The burner was designed and fabricated using CAD-CAM techniques. Computational Fluid Dynamic techniques were used to investigate the performance of the burner. The flames produced by the burner presented in this paper, achieve moderate uniform heat fluxes over relatively large areas. These type of characteristics are specifically favorable for industrial melting processes. The structure of the fuel spray has been investigated experimentally to provide information for CFD modelling and to examine its effect on combustion performance. The spray quality was characterised by SMD measurements. The results are used to analyse burner performance during bench-scale tests as well as during operation in an industrial furnace. Testing was done in a laboratory-scale process-furnace of 0.8 x 0.6 x 2 m. Firing rates up to 0.3 MW were used. Optical and physical probing techniques have been used to measure temperatures, major and minor stable species, droplet-size distributions and velocities. Results show significant reductions in NOx emissions as a result of using oxygen as oxidant. Furthermore, measurements show a high sensitivity of NOx emissions to the details and the physical location of fuel injection. This high sensitivity is due to primarily the interaction of the fuel-spray structure and the time-temperature history in the reacting mixing layer. The current findings been used to optimise the operation of the oxy-fuel burner. Based on this work, measurements have been performed in a commercial borosilicate glass furnace using the oxy-fuel burner. Infrared thermal-imaging, photography, flue-stack analysis, and analysis of furnace-operating parameters have been used to monitor the effects of oxy-fuel combustion on the glass-melting operation. Increased heat-transfer rates have resulted in increased productivity, improved glass quality, decreased fuel consumption and lower NOx emissions. INTRODUCTION Oxygen-enrichment techniques are used in air-fed combustion systems to improve fuel efficiencies as well as to increase productivity (1 , 2). The use of oxygen in place of air results in very marked changes in the scalar field (temperature ·and species) as well as the velocity field. Converting to oxygen as the sole oxidant in the combustion system provides additional benefits such as decreased volumetric requirements of the combustion space, reduced exhaust-gas volumes, elimination of the combustion air-supply systems and has potential for dramatic reductions in NOx emissions. Oxy-fuel firing can achieve this through increased flame temperatures, convective flow-field alterations and improved flame shaping for optimum heat transfer. The high heat-release rates associated with oxy/liquid-fuel combustion which can be - 100 MW/m3 can lead to excessive high heat fluxes resulting in localised overheating if the heat cannot be dispersed quickly enough in the thermal load. Proprietary oxygen-injection techniques have been developed to provide uniform heat-transfer enhancement while avoiding the "hotspotting" problem. The application of these techniques have been extended to oxy-burner technology (3). Essentially, aerodynamically generated low-pressure fields are used to establish uniform heat transfer over large areas. The development of a liquid-fired oxy-fuel burner (the "Globally-Enhanced" burner) is reported here, and the application of this burner to a furnace melting borosilicate (E) glass for stranded fiber production is described. This type of glass melting is particularly notorious for its heat transfer limitations and consequential need for a uniform heat flux. |
