| Research Article |
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Single droplet ignition and combustion characteristics of multi-component heavy oil |
| CHEN Jian, ZHANG Yang, ZHANG Hai |
| Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Energy and Power Engineering, Tsinghua University, Beijing 100084, China |
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Abstract [Objective] Heavy oil is one of the heavy end products of the petroleum refining process and has high energy density, low price, and poor ignition and combustion performance. There is an increasing demand for heavy oil and substantial variation in the composition of heavy oil provided by different suppliers. Understanding the combustion characteristics of heavy oil, especially those related to ignition and burnout, is critical to safely use and effectively tune and control the combustion of heavy oil. The development of heavy oil burners can yield significant insights into this endeavor. The combustion of heavy oil in the furnace is essentially the combustion of a large number of liquid droplets. The study of the ignition and b[KG-*4]urnout characteristics of a single liquid droplet can lead to a better understanding of the combustion of heavy oil. [Methods] The composition of multi-component heavy oil is analyzed, and the single drop experiment is carried out. Considering the need to measure the temperature at the droplet center and take pictures of the static droplet during the experiment, the hanging drop method is chosen for studying single droplets of three multi-component heavy oils at different temperatures. Also, experiments are performed in tubular furnaces with different initial diameters to understand the correlation between the ignition and combustion characteristics of a heavy oil droplet and its composition, ambient temperature, and initial diameter. The droplet ignition delay is defined as the duration between the entrance of the droplet into the tubular furnace and its ignition. All experiments are conducted under visible light to enable observation of the variation in the ignition delay of liquid droplets and the time taken for the burn-out of their volatile components. Then the experiment is repeated under a strong backlight to better observe the change in the droplet radius during combustion. The obtained images are binarized by the MATLAB program, and the 256 brightness levels of the original image are converted into pure black or pure white, respectively, through threshold adjustment. The pure black part denotes liquid droplets and thermocouples. The pixel count of the black regions is determined, the region of interest (ROI) is set manually to exclude the thermocouple wire, and the actual area of the droplet is calculated. The characteristic radius is defined as that corresponding to the equivalent circle area equal to that of the droplet image.[Results] The ignition delay and the burnout time of a heavy oil droplet were found to decrease with increasing temperature and increase with increasing initial diameter. However, given that these variations were not directly related to the proportion of the heavy components of the droplet, it was unreasonable to try to correlate the proportion of the heavy component with the combustion performance of the droplet. Due to the complex mix of components with different boiling points, the expansion behavior of the heavy oil droplets as a function of time during combustion strongly influences their combustion characteristics. The thermogravimetric-mass spectrometry (TG-MS) characterization could explain the frequency and amplitude of expansion. The expansion times of the three samples were consistent with the number of pyrolysis peaks in TG-MS results, and the expansion amplitude was positively correlated with the height of the pyrolysis peaks. [Conclusions] The multi-component heavy oil droplets are found to undergo micro-explosions during combustion due to the complexity of their composition and large differences in the volatilities of the individual components. The intensity of micro-explosion during droplet combustion is defined. It is found to be positively correlated with the proportion of heavy components, the initial diameter of the droplets, and the ambient temperature.
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| Keywords
fuel and combustion
heavy oil
droplet combustion
microburst
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Issue Date: 22 April 2023
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2023,
Vol. 63
