It is important to be able to accurately and reliably measure the concentration of sulfur compounds in liquids, as various chemical reactions may take place that would release sulfur compounds into the atmosphere or onto physical structures around the sulfur-containing liquid. For example, the combustion of diesel fuel typically generates sulfur oxides (SO2, SO3) and sulfuric acid (condensate H2SO4), both of which are components of acid rain. Further, these sulfur compounds have been linked to catalyst deactivation in various aftertreatment components such as diesel particulate filters (DPFs), diesel oxidation catalysts (DOC), NOx trap catalysts, and SCR catalysts. Moreover, sulfuric acid condensation has been linked to severe corrosion of engine components, such as the cooler and piston ring liner components. Such phenomena are found when using both high sulfur (>350 ppm) and low sulfur (15-350 ppm) fuels.
For various reasons, including the sensitivity of aftertreatment components to sulfur compounds, many modern diesel engines are now being designed to use Ultra Low Sulfur Diesel (ULSD) fuel (<15 ppm S). Accordingly, the sulfur level of the fuel source is of utmost importance for optimum machine performance. Examples of known means of detecting sulfur in a wide range of concentrations include ultra-low levels include Flame Photometry Detection (FPD), Inductively Coupled Plasma (ICP) devices, and Monochromatic Wavelength Dispersive X-Ray Fluorescence (WDXRF) spectroscopy, but these methods are more appropriate in the laboratory setting because of the size of the necessary instruments, the duration of test cycles, frequent instrument calibration, and high voltage power requirements. So while sulfur detection in liquids for a wide range of concentrations as well as at levels below 15 ppm is attainable in a laboratory setting, such detection is not feasible in the field or on-board with an accurate, portable, reliable, quick, and inexpensive sensor.
Sulfur organic compounds in diesel fuel can be represented by the formulas R—S—H and R—S—R, where R includes various aliphatic derivatives (saturated or unsaturated), cyclic derivatives, and aromatic derivatives. It is known to those skilled in the art that high sulfur fuels contain predominantly aliphatic and cyclic derivatives, while ultra low sulfur fuels contain mostly aromatic derivatives. Therefore, a sensor operates in wide range of concentrations and should be capable to respond accurately to a variety of sulfur organic species in the liquid.
U.S. Pat. No. 6,716,336 B2 describes an electrochemical sulfur sensor based on an ion conductive ceramic, the sensor being composed of a working (sensing) electrode (porous gold layer) in contact with a liquid (such as fuel), a reference electrode (Ag layer) insulated from the liquid, a reference material (AgS) associated with the reference electrode, and a membrane positioned between the two electrodes. The membrane is in contact with and impermeable to the liquid while it is permeable to an ion that forms a chemical compound with the sulfur species in the liquid. An example is the ion Ag(+); therefore Ag-β-Al2O3 was utilized as an Ag-ion conducting ceramic solid electrolyte membrane in the sensor design. Such a sensor exhibits a change in electrical signal (measured as potential) because of the change in ionic conductivity of ceramic membrane materials in contact with sulfur organics in the liquid. Although such a sensor performs well with the “simulated diesel fuel” composed of mostly aliphatic sulfur organics and thiophene, the sensor disclosed in '336 is not successful when it is used with commercially available diesel fuel. Accordingly, a desire for a fast and inexpensive detection of sulfur level in diesel fuels, or possibly an on-board diagnostic tool for determining the same, persists.