Effect of Degree of Unsaturation on NOx Formation in Partially Premixed Flames of Biofuels

The effect of iodine number on NOx formation in laminar flames of three oxygenated biofuels was studied at an equivalence ratio of 2. Neat soy methyl ester, neat canola methyl ester, and methyl stearate, which had similar energy content and carbon chain length, were tested. The iodine numbers for these fuels varied from 0.5 to 142. Inflame NOx concentration and temperature were measured. The peak NOx concentration occurred in the near-burner region for all biofuels: 404 ppm for soy, 388 ppm for canola, and 123 ppm for the methyl stearate fuel, and did not correlate with the location of the peak temperature. Therefore, the peak concentration in this region was traced to the Fenimore mechanism. Also, the peak NOx concentration was significantly increased with iodine numbers (degree of unsaturation of the fuel molecule). Thus, a strong correlation exists between the chemical structure of the fuel and the NOx emission of the premixed flame.