Experimental investigation of hydrogen enriched natural gas diffusion reactions using preheated air in a hot coflow burner

Abstract

The present paper analyzes experimentally the morphology of a set of non-premixed diffusion reactions of hydrogen enriched natural gas (NG). For the analysis the H2 content was varied from 0 to 100 vol.-% and the mixture issues in a co-flow at variable preheating temperatures, from ambient conditions of 25 °C and up to 400 °C. For the various NG-H2 blends the thermal power input was held constant at 0.8 kW. For processing the images, a bitmap filter was used to isolate visible flame height, visible width variations, and soot free length fraction (SFLF). The results show that visible flame height always decreases when increasing H2 content in NG at any preheating temperature. The effects of air preheating summarize as follows: for NG enriched up to 75 vol.-% H2, the visible flame height reduces when increasing the air preheating temperature from 25 °C to 200 °C, then at 300 °C the visible flame height increases when rising the air temperature up to 400 °C. For the 100% H2 non-premixed flame the visible height presents a linear trend to increase as air temperature rises. Roper's theory for non-premixed flames in circular ports correlates reaction length to air preheating temperature and fuel composition variations, however those trends neglect buoyancy and diffusion phenomena, which results in significant error on predicting the effects of hydrogen addition to NG on the reaction length. A modified correlation is proposed to predict accurately the effects of air preheating and H2 fraction on visible flame height. Finally, a multidimensional scaling analysis shows that hydrogen addition up to 50 vol.-% does not modifies the visible flame morphology significantly when compared to pure NG.