|Combustion, thermal and emission characteristics of gas-fired inverse diffusion flames burning biogas
|Leung, C. W. (ME)
An, L. (ME)
|Hong Kong Polytechnic University -- Dissertations
Heat -- Transmission
Air -- Pollution -- Measurement
|Faculty of Engineering
|103 pages : color illustrations
|This paper presents the results of an experimental study on the thermal and pollutant-emission characteristics of a biogas/methane-fired inverse diffusion flame (IDF). The IDF burner used in the experiment has a central air jet surrounded circumferentially by twelve outer fuel jets. The biogas was premixed by methane (CH₄) and carbon dioxide (CO₂). Two different components of biogas were prepared, one is 50% CH₄ and 50% CO₂ while the other is 75% CH₄ and 25% CO₂. Besides, a control group was investigated by burning 100% pure methane. The thermal characteristics of inverse diffusion flame was measured with the flame impinging vertically upwards on a horizontal copper plate. The heat fluxes at the stagnation point and the radial distribution along the impingement plate were measured with a heat flux sensor while the temperatures were measured with thermocouples. The effects of CO₂ concentration of biogas, Reynolds number, overall equivalence ratio, and nozzle-to-plate distance on the flame and heat flux distribution were investigated. The temperature and heat flux were calculated within a radial distance of 30 mm starting from the stagnation point of the flame, and air jet Reynolds numbers were chosen at 1000, 1500, 2000, 2500, 3000, 3500 and 4000. For two kinds of biogas, all the operation conditions were measured under both equivalence ratios of 2 and 3 with three different nozzle-to-plate distances of 60mm, 90mm and 120mm (corresponding to the H/d = 10, 15 and 20). But for 100% pure methane, equivalence ratios were chosen as 0.7, 1.0, 1.3, 1.6 and 2 while the nozzle-to-plate distances are 60 and 90 individually (corresponding to the H/d=10 and 15). Besides, four different air pollutants emitted from the flame: HC, CO₂, CO and NO₂ were measured. And emission index of these four pollutant emissions are calculated. Comparing the results, heat flux and flame temperature distribution were affected by the CO₂ concentration of biogas, equivalence ratio, Reynolds number and nozzle-to-plate distance together. In conclusion, addition of CO₂ may lead to decrease of temperature and heat flux in general. Higher equivalence ratio leads to occurrence of higher temperature and heat flux when the other parameters maintained as constant but too high equivalence ratio may also lead to the lift phenomenon. The maximum temperature and heat flux occurs at the stagnation point when the plate is above the flame, whereas the temperature heat flux is low around the stagnation point when the flame is truncated by the plate. As for air pollutant emissions, addition of CO₂ into biogas will almost reduce all four pollutant emissions. Besides, HC is rare when the combustion is complete while CO increases with incomplete combustion. Too high or too low equivalence ratio will both lead to uncomplete combustion. Equivalence ratio has no significant influence on emission index. However, CO₂ emission index is quite high comparing with other emissions which should be reduced in application.
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