Spectroscopy is a powerful tool with a wide range of applications that can protect the environment by monitoring and regulating air pollution.
Danish multinational Danfoss IXA has developed an emissions analyzer in the ocean based on ultraviolet (UV) absorption spectroscopy to monitor nitrogen oxides (NOx), sulphur dioxide (SO2) and ammonia (NH3) emitted from cargo ships. The optical monitoring equipment is located inside the ship's exhaust system and is exposed to harsh environments with extreme temperatures, vibrations, and corrosivity, which place severe environmental demands on the spectroscopy system.
Why monitor emissions from cargo ships?
Marine emissions from international shipping vessels cause premature deaths from lung damage and cardiovascular disease in people around the world. The number of heart, lung and lung cancer deaths caused by shipping emissions is estimated to be as high as 60,000,000 per year globally. Not only is marine vessel emissions a serious problem that affects human health, but it also damages marine and terrestrial ecosystems.
The International Maritime Organization (IMO) and the U.S. Environmental Protection Agency (EPA) have established Emission Control Areas (ECAs) in many of the nation's oceans with strict emission regulations - without which ships cannot enter many major ports.
Without analyzers like those developed by Danfoss IXA, for example, authorities have no other convenient, reliable way to monitor ship emissions and enforce these regulations. While there are many local and regional initiatives aimed at limiting ship emissions, enforcing these policies is extremely difficult. The Spectrum-based Marine Emissions Analyzer is a powerful tool capable of accurately monitoring ship emissions in real time.
UV Spectroscopy System
The basic principle of spectroscopy is that substances have a unique absorption spectrum and are able to absorb different wavelengths of light depending on their atomic and molecular composition.The UV Spectroscopy System from Danfoss IXA consists of a high-intensity UV light source, a UV spectrometer, and UV-enhanced optical components such as optical fibers, lenses, and planar mirrors. In order to understand how different wavelengths are absorbed and in this way determine the composition of the exhaust gas, the spectrometer spatially separates the broadband emission of the light source onto a 1D detector array, which measures the entire UV spectrum simultaneously.
While Danfoss IXA's system does not use monochromators for wavelength isolation, many spectroscopy systems use monochromators for wavelength isolation. In these cases, light from a UV source enters the entrance slit of the monochromator, where a dispersive element (such as a diffraction grating or prism) breaks the light down into the component wavelengths it contains (see Figure 1).
Image Figure 1: The test wavelength of a spectrometer, which can be fine-tuned by separating the broadband emission onto a 1D sensor array, or by changing the angle of the diffraction grating or prism inside the monochromator. (Image credit: Edmund Optics)
The outgoing slit of the monochromator blocks all wavelengths, and only a narrow band of light that passes through the exhaust sample passes through the slit. Changing the angle of the diffraction grating or prism changes the wavelengths that pass through the outgoing slit, allowing fine tuning of the test band. The light passing through the exhaust sample is then directed to a detector to determine the absorption that occurs; the molecular composition of the exhaust gas is then calculated from the absorption results.
For monochromators using diffraction gratings, the notch frequency of the grating is usually measured in notches per millimeter. A higher notch frequency improves optical resolution but results in a narrower range of available wavelengths; conversely, a lower notch frequency results in a wider range of available wavelengths, but at the expense of optical resolution.
Environmental Requirements
The development of such systems is very challenging due to extremely high temperature and pressure requirements. High temperatures can cause optics to fail due to melting and thermal stress, which severely limits the types of optical materials that can be used. High temperatures can also cause adhesives in optical components to outgas and contaminate the system. The system is exposed to temperatures up to 500°C, so its high-pressure requirements make sealing the optical system critical. The need for optics to transmit UV light with little or no absorption also limits the available optical materials.
UV degradation of the optics
Another challenge facing the project is that UV optics tend to have limited lifetimes, largely due to contamination from high-power UV photons interacting with the environment and UV light damaging the coatings and substrates of the optics. Both of these effects degrade the performance of optical components over time.
Harmful materials may be deposited on the surface of the optics when high-power UV light interacts with particles, water vapor, organics, and other contaminants in the system. Exhaust and other airborne contaminants commonly cause carbon deposits on optical surfaces. Figure 2 shows an example of UV-induced dendritic growth of contamination.
Image Figure 2: An example of contamination induced by exposure of an uncoated fused silica window to UV light. This image was taken after 6 weeks of exposure to a UV laser at approximately 3W, which is different from the use of the gas analyzer in the Danfoss IXA, but it gives an indication of the type of UV contamination that can occur.
Interaction with the gases surrounding the optics can also lead to deposition of contaminants, so any exhaust gases entering the system are a source of contamination. Photon energies at UV wavelengths less than 400 nm are close to the same as the bond energies of the surrounding molecules, which allows the UV light to break some of these bonds. This produces other ions and molecules that can contaminate optical surfaces.
Due to the process of optical fatigue, the coatings and substrate materials of UV optics devices themselves are also susceptible to degradation over time when exposed to high power UV light. Heavy use over time can cause them to degrade and lead to discoloration or other changes in the material. Their refractive index can be modified to produce a lensing effect that can increase localized intensity. Self-trapped excitons can also be formed, which leads to the accumulation of absorption centers.
As a result of these effects, UV optics may need to be replaced over time, but proper sealing, washing and cleaning can mitigate these effects.
The harsh environments that the Danfoss IXA Gas Emissions Analyzer has to adapt to have posed many challenges to the optical and opto-mechanical design of the system; however, the device is proving to be a success and is currently helping to monitor emissions from thousands of ships worldwide.
This is a great victory for the environment - a step towards minimizing NOx, SO2 and NH3 emissions from international shipping. Any reduction in this pollution helps to reduce the number of deaths from heart and lung diseases caused by shipping emissions each year.
When designing an optical system to operate in harsh environments, discuss the specific environmental requirements with the optical component manufacturer. The optical component manufacturer should be able to guide you through the key considerations, clearly explain any trade-offs that may need to be made, and ensure that your system operates as needed.
