PAHs (Polycyclic Aromatic Hydrocarbons) are a group of over 100 different chemicals that occur together as mixtures. PAHs occur naturally in petroleum and are produced as by-products of fuel combustion. The EPA has identified 16 PAH compounds as priority pollutants, which includes phenanthrene, pyrene and fluorene.
Traditionally, the shipping industry has used high sulfur bunker fuel resulting in high particulate matter, SOx, and PAH emissions. In order to reduce air pollution from SOx emissions, the International Maritime Organization (IMO) has put a global cap of 0.5% on sulfur content in marine fuel, and this regulation comes into effect Januart 1, 2020. In effect, the shipping industry has two options: switch to expensive low sulfur fuel or implement relatively lower cost exhaust cleaning systems. Exhaust cleaning systems use gas scrubbers to reduce emissions, which clean a wide range of pollutants out of the exhaust gas, including SOx and PAHs. After scrubbing, the wash water is treated and monitored for PAHs prior to being discharged into the sea to avoid pollution shift from air to the water.
To prevent this pollution shift, IMO guidelines recommend that PAH concentration in washwater, along with turbidity, pH and temperature be continuously monitored and recorded when exhaust gas cleaning systems (EGCSs) are in operation. In addition, as PAHs are also found naturally in petroleum, PAH monitoring ensures that unburned oil or hydrocarbons do not enter the sea.
Since phenanthrene is the most prevalent of the 16 EPA PAHs found in the vessel washwater systems, the IMO Guidelines set the washwater criteria for PAH in phenanthrene equivalents, rather than measuring each of the 16 PAHs. The maximum continuous PAH concentration in the washwater should not be greater than 50 µg/L PAHphe (phenanthrene equivalence) above the inlet water PAH concentration.
Low concentration levels of PAHs such as phenanthrene can be measured using fluorescence spectroscopy at 255 nm excitation. In fluorescence spectroscopy, the emission intensity (signal) is directly proportional to the concentration of the fluorescent compound over a wide range of concentrations. The emission intensity is also directly dependent on the intensity of excitation, so the higher the intensity of the light source at 255 nm, the more sensitive the detection.
Fluorometers operating in the deep UV wavelengths have traditionally used xenon flash or deuterium lamps because they deliver sufficient light in these wavelengths. However, these sources have more complex circuitry and, therefore, a higher cost of ownership. With the release of Optan LEDs, sensor manufacturers can make use of the highest intensities in the deep UV wavelengths, due to the lattice matched aluminum nitride substrate. The high light output of Optan LEDs increases sensor sensitivity and enables measurement of low detection limits. Another advantage is the high light output enables low current operation and in turn, significantly longer lifetime and replacement cycle with LEDs, which reduces cost of ownership. In addition, the use of an LED simplifies system design significantly, further lowering total cost of ownership for the marine operators.
Read this case study to learn how Chelsea Technologies Group benefited from switching to Optan LEDs in their Uvilux sensor.