Biofuels—principally biodiesel and bioethanol—have become increasingly important as renewable fuels with a potentially lower carbon footprint than fossil fuels. With numerous national and international standard specifications for fuel quality, and the potential for variability between biofuels produced by various methods from numerous feedstocks, fast and reliable analysis of biofuels and biofuel blends is critical. Fourier transform infrared (FTIR) spectrometry is particularly well-suited to the measurement of several important fuel parameters, such as the rapid determination of critical impurities at subpercent levels in both biodiesel and bioethanol. The concentration of biodiesel in diesel fuel—whether it is a desired blend component or a contaminant—can be accurately quantified by FTIR at concentrations as low as tens of ppm. Finally, the specific biological origin of biodiesel affects its properties, and FTIR analysis allows discrimination between biodiesel samples from various feedstocks such as palm, soy, and rapeseed.
Impurities in bioethanol
Figure 1 – Spectrum Two FTIR spectrometer.
Bioethanol is produced by fermentation of sugars, and purification steps are required to meet the requirements for use as fuel, which are set by international standards such as ASTM D4806 and EN 15376. At present, the specified test procedures are time-consuming chromatographic and titrimetric methods, and FTIR could provide a rapid alternative with sufficient sensitivity to meet the required detection limits for methanol, water, C3–C5 alcohols, and gasoline denaturant.
A feasibility study was conducted by preparing 60 mixtures of ethanol with water (0–1%m), methanol (0–1%m), 1-propanol (0–1.7%m), 1-butanol (0–1.7%m), 1-pentanol (0–1.7%m), and petroleum spirit (0–7%m). The spectra were measured on a Spectrum Two FTIR spectrometer (PerkinElmer, Buckinghamshire, U.K.) using a 0.1-mm liquid flow cell with BaF2 windows (see Figure 1). The flow cell allowed samples to be injected and drained to waste rapidly, for a total analysis time of ~2 min per sample, with negligible carryover.
Figure 2 – Cross-validaton prediction plots for quantitation of impurities in ethanol.
PerkinElmer Spectrum Quant+ software was used to build and cross-validate full-spectrum PCR models for all of the analytes. The results of the cross-validation are shown in Figure 2.
Table 1 – Summary of FTIR ethanol analysis results and comparison against specified impurity limits
As shown in Table 1, the achievable detection limits are within the specified limits; therefore, FTIR is a feasible method for this analysis. In practice, real-world samples will contain a broader range of impurities and it will be important to use a representative calibration set and an independent validation.
Biodiesel blend concentration
FTIR is well suited to the measurement of biodiesel content in diesel. Biodiesel consists of fatty acid methyl esters (FAMEs), and the carbonyl group has a strong absorption band that is free from interference by the mineral diesel matrix. Recognizing this fact, there are two standard test methods for FAME in diesel that utilize FTIR: ASTM D7371 and EN 14078. The former uses an attenuated total reflectance (ATR) measurement that trades a more involved calibration procedure for slightly improved ease of measurement, while the latter uses a transmission measurement that is straightforward to calibrate and allows excellent sensitivity for low concentrations. The PerkinElmer FTIR solution for EN 14078 can easily be scaled from portable instruments with work-flow-driven touchscreen software for ease of use in smaller labs, to high-performance autosampling systems capable of analyzing more than 100 samples per hour.
Figure 3 – Spectra of low-concentration biodiesel samples and calibration graph. Spectra were measured on a Spectrum Two instrument.
Transmission measurements similar to EN 14078 can be used to check for the presence of trace amounts of FAME in fuel that should be FAME-free. ISO 8217 (2010) recommends that marine distillate fuels should not contain FAME, with a contamination limit of 0.1%. FTIR is easily capable of measuring this concentration, and detection limits of tens of ppm can be obtained when a representative reference spectrum of uncontaminated fuel is available for subtraction (see Figure 3).
Biodiesel feedstock identification
Biodiesel is produced from a wide range of naturally occurring fats and oils. The fatty acid distribution of the original oil is retained in the biodiesel, and thus the physical and chemical properties of the biodiesel have some dependence on the feedstock used. In particular, the cloud point is affected by the fraction of saturated chains, as these begin to crystallize at higher temperatures.
Figure 4 – Spectra of biodiesel from several feedstocks, showing the changes that arise from the varying degrees of unsaturation.
Spectra of biodiesel from three of the most common feedstocks (palm, rape, and soy oils) were obtained using a PerkinElmer FTIR spectrometer with a UATR single-reflection diamond accessory. Clear differences can be seen in regions of the spectrum corresponding to alkene functional groups. The intensity increases in the order palm < rape < soy. This observation is consistent with the data in Table 2 derived from tabulated chain distributions for these oils. FTIR spectroscopy thus provides a very quick way to check the provenance of a biodiesel sample (see Figure 4).
The increasing global emphasis on biofuels—fuels produced from renewable biological sources—results in a growing need for rapid and reliable analysis of pure biofuels and biofuel blends. With modern instrumentation such as the Spectrum Two and OilExpress 4, FTIR spectrometry has a valuable role to play both as an established method for biodiesel blend analysis, and as an emerging technique for analysis of impurities at low concentrations.
Ben Perston is an Application Scientist, PerkinElmer, Chalfont Rd., Seer Green, Buckinghamshire HP9 2FX, U.K.; tel.: +44 (0) 1494 679 248; e-mail: Ben.Perston@perkinelmer.com.