Visible and NIR Spectroscopy to assess biodiesel quality: Determination of alcohol and glycerol traces

Biodiesel quality control is of relevant importance as biodiesel properties influence diesel engine performance. In the present work, the benefits of the use of visible and near-infrared Spectroscopy (NIRS) as a technique for screening undesirable contaminants, i.e. methanol and glycerol content in biodiesel are presented. Excess of methanol decreases heating value and flash point and increases carbon deposits, while the presence of glycerol may cause injector tip coking and deposits in the combustion chamber. Biodiesel samples contaminated with different amounts of methanol and glycerol were scanned by NIRS. Their NIR spectra were acquired at 2-nm intervals over a wavelength range from 400 to 2500 nm (visible plus near-infrared regions). First derivative of the spectra were calculated and correlated to the raw optical data by means of modified partial least-squares (MPLS) regression. First derivative equation of the optical data, pretreated by standard normal variate (SNV) and De-trending (DT) transformations, showed a coefficient of determination r2 in the cross-validation step of 0.99 and 0.81, for the samples contaminated with methanol and glycerol, respectively. Also, the standard deviation to standard error of cross-validation ratio (RPD) was 10.0 and 2.5, respectively. These statistics are indicative of the high capacity of prediction of the equations for methanol content and acceptable for glycerol content. Visible spectra also showed differences related to the samples, thus indicating it could serve to determine the presence of these contaminants. The use of NIRS technology provides a trustworthy and low-cost method to determine the presence of undesirable amounts of methanol and glycerol. It also offers an important saving of time (each analysis requires less than two minutes).