Technology: Vapour tracker goes to sea

 作者:芮癃觯     |      日期:2019-02-28 08:12:01
By RICK GOULD Air pollution from oil tankers which load up at sea can now be monitored by a technique similar to radar, which uses laser beams instead of radio waves. Developed by scientists from a British company called Spectrasyne, the system uses LIDAR – which stands for laser interferometry detection and ranging – to measure the amount of gaseous hydrocarbons discharged from the tanks as they fill. The technique has been adapted from one used to monitor emissions at oil refineries on dry land. The development should allow oil companies to control the evaporation of hydrocarbons more effectively than has been possible with previous methods, which relied on estimations derived from mathematical formulae. The LIDAR technique, which can measure the total emissions from tankers as they load, has proved to be cost-effective: in its first use, at a land-based oil refinery in Goth-enburg, Sweden in 1989, the amount of vapour leaking was found to be more than five times as much as predicted. The emissions were subsequently cut by several thousand tonnes annually. Spectrasyne, based in Basingstoke, uses a technique known as differential absorption LIDAR, or DIAL. Normal LIDAR estimates distances by scanning a single laser beam over an area being monitored, and detecting interference effects from laser wavefronts that are reflected back by distant objects. With DIAL, a dual beam at two different frequencies is scanned over an area, and particles and aerosols in the atmosphere reflect the beams back to a detector. Pollutants are monitored by tuning one beam to an absorption frequency of a target pollutant: if the pollutant is present, it will absorb the beam, so less laser energy is reflected back to the detector. Integrating the results for a whole area can produce three-dimensional emission profiles of air pollutants. This is the first time DIAL has been used to monitor emissions at sea. ‘For the technique to work, you need to know exactly where the laser beam is pointing,’ says Spectrasyne’s technical director, Jan Moncrieff. Until now, DIAL required a stationary platform for the laser projector and detector; otherwise wind and wave motions affected the direction of the outgoing laser beam and the returning signal. To overcome this problem the scientists developed an orientation sensor which detects motion and tilt, and mounted the whole system on a barge. The sensor links up with the data acquisition system for the laser beams, enabling it to compensate for changes in the position of the barge. The new technique will be ideal for monitoring tankers with several tanks and vents. Oil companies can already monitor emissions by measuring the losses at each vent, but this is at best an estimate because there are many points besides the vents where leaks can occur. It can also take up to 24 hours to load a marine tanker,