46500-AC9
Oxygenated Fuels in Flames
Our research during the project period has focused on the propensities of oxygenated hydrocarbons to form soot in flames representative of transportation devices such as diesel engines and gas turbines. Sooting tendencies have traditionally been characterized by the heights of smoke point flames, which are coflow nonpremixed flame of the test species with the fuel flowrate adjusted so that the flame is on the boundary between emitting and not emitting soot particles to the ambient air through its tip. This definition has many drawbacks, not least that it requires the subjective determination of smoke point, so a few years ago proposed a new definition: the maximum centerline soot volume fraction of a methane/air coflow nonpremixed flame with a small quantity of the test compound doped into the fuel. We have applied this definition, which we call Yield Sooting Index (YSI), to aromatics and demonstrated that it can be measured more accurately and precisely than smoke height. Further advantages of YSI are that it requires only a small quantity of the test compound (< 1 g), which greatly expands the range of compounds that can be tested, and it characterizes the sooting tendency in the environment of a methane flame, which is more representative of devices burning petroleum-based fuels than pure fuel flames of most aromatic compounds would be.
In our recent experiments we have made preliminary measurements of YSI for a range of oxygenated hydrocarbons. The results demonstrate that YSI is applicable to oxygenated compounds, which have low sooting tendencies, as well as aromatic compounds, which have high sooting tendencies. To produce a measurable change in soot concentration beyond the background soot from the methane, the concentration of the test compounds had to be increased from 400 ppm for the aromatics to 4000 ppm for oxygenates. Nonetheless, this is still a low enough concentration that the temperatures and overall flame structure of the flame are not affected by the dopants.
The most important result of the new measurements is that the sooting tendencies of oxygenates are strongly dependent on the specific details of their molecular structure. For example, the measured YSI’s for the five isomers of the C5H12O2 esters vary by more than a factor of two, from 6.8 for methyl butanoate to 13.9 for propyl acetate. These compounds all have the same ratio of carbon to oxygen atoms and the same molecular weight. Esters are the primary components of biodiesel fuels, which have been proposed as fuel extenders in part due to their ability to suppress soot emissions. Our results show that their effectiveness in reducing soot will depend strongly on the specific structure of the esters.
Future experiments will systematically measure YSI for a much larger number and range of oxygenates, such that detailed correlations can be established between sooting tendency and fuel structure. Species measurements will also be performed to understand the chemical mechanisms underlying these correlations. These correlations will allow optimum additives to be identified without extensive and expensive engine testing.
