Effects of Equivalence Ratio on Combustion Characteristics of Laminar Partially Premixed Flames of Petroleum-Biofuel Blends

Abstract

In order to provide energy independence and control pollutant emissions during combustion, alternative fuels are being developed. Biodiesel (fatty acid methyl esters) has received, and continues to receive, considerable attention for its potential use as an augmenting fuel to petroleum diesel. Its advantages include decreased net hydrocarbon, carbon monoxide, and particulate matter emissions, and fuel properties similar to petroleum diesel for ease of use in diesel engines. Its disadvantages include poorer cold flow characteristics, lower heating values, and mostly higher emissions of oxides of nitrogen. In addition, using pure biodiesel in an engine can clog fuel filters due to its poorer cold flow characteristics. Because biodiesel is a strong solvent, it will probably loosen debris in pipes and tanks, sometimes degrading rubber hoses. Biodiesel blends with diesel fuel are preferred in such conditions.

Biodiesel blends with petroleum diesel at a mixing ratio between 2 and 20 vol. % are widely offered as automotive fuels. The target for the future is to bring this ratio to a higher percentage, in order to increase the share of renewable energy in transport. Knowledge of the combustion and pollutant emission characteristics is important in the application of biofuels and their blends. There is, however, limited evidence on the effects of such blends on the combustion and emissions of diesel engines not originally designed to operate on biodiesel blends. Biofuels, such as canola methyl ester (CME) & soy methyl ester (SME), have considerable potential for use as fuels in internal combustion engines. In the current study to understand the effects of equivalence ratio on the combustion properties of petroleum-biofuel blends, partially-premixed laminar flames of prevaporized blends were investigated. A laminar flame environment was chosen to simplify the fluid mechanics. The primary objective of this study was to clarify controversies and discrepancies in literature that existed and clearly define the cause(s) of soot and NOx formation in biofuel blends on a chemical basis. The equivalence ratios were chosen to simulate the partial premixed to non-premixed flame combustion zones that exist in the far-injector regions in diesel engines. The documented combustion characteristics included inflame species concentration, inflame temperature, global emissions, global radiation, OH, and CH radicals, and soot volume fraction. To investigate the primary mechanism(s) which would contribute to soot and NOx formation and their interactions for biofuel blends on a chemical basis alone was the goal of the project.

The fuel was vaporized by injecting into a hot air stream. The resulting flame was laminar whose characteristics were dependent on the chemistry of the fuel alone. Three blends of CME with petroleum-based diesel & three blends of SME with petroleum-based diesel were used with 25, 50 and 75% volume concentration of the biofuels respectively. The equivalence ratio was altered by changing the air flow rate. The measured radiative heat fraction significantly increased with increasing equivalence ratio (1.2 to 7). A decrease in the soot volume fraction was observed as the volume percentage of biofuel was increased in the blend. It was found that the NOx emissions and the flame temperature decreased as the equivalence ratio was increased for all fuels tested. The biofuel flames produced the highest emission index of NOx, which decreased as the volume percentage of biofuel was decreased in the fuel blend. In contrast, the CO emissions increased as the equivalence ratio was increased for all fuels tested. CO emissions decreased as the volume percentage of biofuel was increased in the blend due to the presence of oxygen molecule in the biofuel, and consequently lower amount of soot was formed.

A high level of correlation between temperature, soot, and radiation was observed. As the equivalence ratio was increased, the soot content in the flames became larger, leading to larger flame radiation and lower flame temperatures. The biofuel flames behaved similar to petroleum fuel flames, and the quantitative variations are documented in the study. To determine the dominant route of soot and NOx formation in flames of the six biofuel blends, PLIF measurements of OH and CH radicals were carried out. Close to stoichiometry, flames from all fuels produced peak OH concentration fields and peak temperatures. Also, it was observed that residence time increased with NOx concentration. These results indicated the dominance of the thermal (Zeldovich) mechanism for all fuels at this condition. The results of the OH concentration and the soot concentration for all fuels tested at near stoichiometry condition shows that, OH radical dominated the soot oxidation process.

Numerical analysis with surrogate fuels (n-heptane and methyl decanoate) was performed with FLUENT software to predict temperature and concentration fields to substantiate the experimental results. Experimental and the numerical model values for temperature profiles showed that n-heptane and biodiesel blend surrogate produced results within experimental uncertainties and meets the criteria for the formation of NO by the Zeldovich mechanism.