IN-LINE FUEL SULFUR ANALYZER FOR IMPROVED MANAGEMENT OF DESULFURIZERS FOR FUEL CELL AUXILIARY POWER

Combat vehicles have unmet needs for auxiliary power units (APUs) that reduce fuel consumption and offer improved operational effectiveness and soldier safety. Adoption of fuel cell APU technology is hindered by requirements for fuel desulfurization and reforming. Both solidoxide and polymer-electrolyte-membrane fuel cells are poisoned by sulfur compounds in jet fuels. To more efficiently manage desulfurization processes, improved in-line methods of determining the sulfur content of fuel are required. Mainstream Engineering is developing a compact, in-line analyzer for measuring total sulfur in jet fuel for fuel cell applications. This analyzer enables less frequent desulfurizer regeneration events when compared to scheduling regenerations based on the assumed worst-case sulfur limit. The measurement technique uses multivariate analysis of Raman intensity spectra, which offers calibration robustness, contaminant detection, and identification of fuel type. INTRODUCTION Fuel cell auxiliary power units (APUs) would significantly reduce military fuel consumption and lead to improved operational effectiveness and solider safety. We estimated that a fuel cell APU could result in up to a 90% reduction in fuel consumption during idling for large combat vehicles [1, 2]. Considering the high fully burdened cost of delivered fuel (ca. $15/gal), the potential savings in fuel expenditures is substantial [3]. Both solid oxide fuel cells (SOFCs) and high-temperature polymer electrolyte membrane (PEM) fuel cells are poisoned by sulfur compounds. Current desulfurizers are regenerable, but the desulfurizers are forced to assume the worst-case fuel sulfur concentration at all times. The sulfur level in JP-8 is capped at 3,000 ppm (0.3 wt %) per the military fuel standard MIL-DTL-83133H [4]. Figure 1 shows the distribution of sulfur levels in fuel supplied by the Defense Logistics Agency in 2009 [5]. As shown, the peak in the distribution is at 1,000 ppm and the distribution is broad. The mean sulfur content varies somewhat from year to year; recently, the mean has been approximately 750 ppm, one fourth what must be assumed by current desulfurizers. Sulfur compounds in JP-8 take several forms, including mercaptans (i.e., thiols), sulfides, disulfides, thiophenes, and benzothiophenes. Of these sulfur compound classes, by far the most prevalent species are various benzothiophenes because they are the hardest to remove by conventional refinery desulfurization processes. Most benzothiophenes in jet fuel have two or three methyl groups attached to the rings as described by Lee and Ubanyionwu [6]. Mercaptans, in particular, are capped at 20 ppm by the military standard and are a minor contributor to total sulfur. Quantification of the total sulfur in jet fuel spectroscopically is challenging because the sulfur is present as an assortment of organosulfur compounds in a complex hydrocarbon matrix. Because desulfurizers are regenerated based on the assumed worst-case sulfur concentration and not the actual sulfur concentration, there is a potential for up to a four-fold average reduction in regeneration frequency by using an inline sulfur analyzer for desulfurizer control. Regeneration is frequently done at high temperatures, incurring significant parasitic losses. A polisher is often needed after the adsorbent bed to get the sulfur down to the very low levels required for high-temperature PEM fuel cells. The polisher might not be regenerable. Also, desulfurization hardware presents a packaging problem. Centeck [1] reported that 10– 20% of the current developmental JP-8 fuel cell APU space claim is consumed by the desulfurizer. The objective of this program is, therefore, to provide an intelligent means to detect the real-time fuel sulfur concentration, allowing less frequent desulfurizer regenerations or downsizing of the desulfurizer for the same regeneration frequency. The research reported in this document/presentation was performed in connection with contract/instrument W911QX-13-C-0005 with the U.S. Army Research Laboratory. The views and conclusions contained in this document/presentation are those of the authors and should not be interpreted as presenting the official policies or position, either expressed or implied, of the U.S. Army Research Laboratory or the U.S. Government unless so designated by other authorized documents. Citation of manufacturer’s or trade names does not constitute an official endorsement or approval of the use thereof. The U.S. Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation hereon. Proceedings of the 2014 Ground Vehicle Systems Engineering and Technology Symposium (GVSETS) Figure 1. Sulfur in JP-8 distribution from 2009 [5] FUEL PROPERTY ANALYZER DEVELOPMENT Fuel Samples The military procures JP-8 fuel from across the world for local and global use. Fuel that conforms to the military specifications must contain less than 3,000 ppm sulfur, but the average is much lower, 750 ppm, with a wide distribution. In addition, all the global regions sampled in 2009 reported samples ranging from near 0 ppm to over 1,500 ppm sulfur [5]. This sulfur variability, the degradation of fuel, and the retrieval and use of captured fuel with unknown sulfur content precludes a simple sulfur test at the place of origin to tune the desulfurizer. A representative set of about 400 JP-8 fuel samples that ranged over 0–2,900 ppm sulfur was collected. As many samples as possible were obtained to maximize the diversity in fuel composition. However, because of the nature of the fuel distribution, the majority of the set of JP-8 samples had less than 1,500 ppm sulfur. In addition to sulfur, 35 other fuel properties were available (e.g., cetane number, flash point) for on average 60% of the samples. All samples conformed to military specifications and ASTM standards. Additional samples were obtained by collecting Jet-A from local airports, doping jet fuel with methylbenzothiopehenes, and desulfurizing JP-8, which brought the total number of fuel samples to over 500. Raman Spectroscopy of Jet Fuels Raman spectroscopy probes rotational and vibrational modes in molecules. The sample is illuminated with a laser beam, and a detector is used to measure the intensity and wavelength of inelastically scattered light. Raman spectroscopy provides a good characterization of compounds with polarizable bonds (e.g., C–C, C–S). The main sulfur compounds in JP-8, thiophene and benzothiophene, can be identified with this technique because of their aromatic groups and strong carbon–sulfur bonds. Raman intensity spectra were collected by a portable Raman spectrometer (200–2780 Δcm, 2.7 Δcm resolution). A fuel sample was excited with a fiber-coupled 785 nm diode laser (350 mW), and scattered light was collected with a fiber-coupled Raman probe (7.5 mm focal length). The probe was inserted into a sample holder that blocked all stray light and fixed the distance to the sample during measurement. A single, ten-second scan was taken of the sample, and the spectra was not smoothed or preprocessed at the time of measurement. An example of the relevant portion of a JP-8 fuel spectrum that has been baseline corrected is shown in Figure 2. Many distinct peaks vary between the samples and can be analyzed for chemical information. Figure 2. Baseline-corrected Raman spectra of JP-8 fuels Chemometric Analysis of Raman Spectra Chemometrics is the use of mathematical and statistical methods to provide maximum relevant chemical information by analyzing chemical data [7]. Although chemometrics of Raman and IR data have been used to predict sulfur in other hydrocarbons [8–10] and for various other fuel properties [11–15], it has not been used to predict jet fuel sulfur content with the desired accuracy and reproducibility for a predictive sensor [16, 17]. We have previously shown that near-IR absorption spectroscopy can be used for fuel sulfur quantification, although with less accuracy compared to the Raman spectroscopic method [18]. Using chemometric tools, large data sets are analyzed with statistical methods to determine linear combinations of independent variables (absorbance at various wavelengths) that best describe the response (sulfur). Regression methods such as principal component regression (PCR) and partial least squares (PLS) determine these “latent variables” (LVs). The LVs are by definition orthogonal and uncorrelated. Typical multivariate calibrations generated from PLS or PCR reduce the dimension of the problem from thousands of discrete absorbances in each spectrum to less than ten LVs. This multivariate approach has significant advantages over the 0.5 10.7 26.3 38.1