| Summary: | 碩士 === 國立成功大學 === 機械工程學系碩博士班 === 100 === Biofuel are gaining increased public and scientific attention, driven by shortage of fossil fuels and greenhouse gas emissions. Biofuel have different physical and chemical characteristics in comparison with fuel oil used in general industrial applications. In order to maintain the same heating and emission characteristics, it is not recommended to burn biofuel directly in the industrial boilers or furnaces, but to burn them as a emulsion of biofuel and fuel oil instead. This research aims to examine the combustion characteristics and pollutant emissions of burning an emulsion of biofuel and fuel oils. This thesis was carried out with three main studies: (1) mixing stability between biofuel and diesel/heavy oil; (2) experimental study of single suspended droplet evaporation; (3) biofuel and heavy-oil emulsion co-firing tests in furnace. Based on these experimental studies, the feasibility of co-firing biofuel with fuel oils can be evaluated to conclude with some strategies for burning biofuel. The experiments results are as described below.
We found that the biofuel was difficult to be mixed with diesel. The emulsion maintained well-mixed no longer than 5 minutes even after adding surfactant. And for the emulsion of biofuel and heavy-oil it was difficult to determine the mixing stability, due to indistinguishable color between each other even though we observed via the optical microscope. In single suspended droplet experiment, the increasing of the heating temperature would accelerate the evaporation rate. In the case of pure biofuel and heavy oil, there were several additional phenomena not found in the experiments using pure diesel. Micro-explosion and random behavior occurred during the experiment. The increasing of temperature caused the random behavior and micro-explosion to occur frequently, due to large amount of water content and impurities in the biofuel. And also ignition occurs at 500 OC in the cases of biofuel/diesel oil emulsion and also biofuel C3/heavy oil at higher biofuel content in the emulsion.
The results of furnace tests indicated the 2.5% biofuel co-firing ran smoothly in the long-term test, with the similar temperature distribution and emission compared to pure heavy oil test. However, the 5% and 10% biofuel co-firing tests were extremely unstable under fuel-preheated condition. Each case of the experiment was tuned to meet the minimum excess oxygen requirement for complete consumption of fuel therefore CO emission level was kept near-zero. NO emission level decreased with biofuel addition, which can be attributed to large amount of water content lowering the combustion temperature. SO2 concentration in the flue gas was directly associated to sulfur content in the fuel we had supplied, which was coincided with the test results for the cases of 2.5% biofuel co-firing and burning pure heavy oil alone. In order to prevent unstable co-firing characteristics, it is suggested to use the separate burners for realistic commercial application of co-firing biofuel.
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