Rick Gould looks at ways to capture and accurately measure volatile emissions in the atmosphere
In 1988, a team of UK scientists used a novel instrument equipped with a powerful laser to scan the air high above and around an oil refinery in Gothenburg. The team set up the instrument, known Differential Absorption Light Detection and Ranging (DIAL), to measure Volatile Organic Compounds (VOCs). These chemicals can be hazardous, a danger to human health and harmful to the environment. When the scientists combined the measurements with meteorological data, the technique produced graphical profiles of fugitive emissions of VOCs, or leaks, from the refinery.
British Petroleum (BP), the refinery operator at the time, typically used emissions factors to estimate fugitive emissions of VOCs, predicting that leaks would add up to about 700 tonnes annually. Using DIAL suggested otherwise – the results were up to 20 times higher. DIAL is one of two techniques the European Commission (EC) now recommends to measure fugitive emissions under new EC legislation for oil and gas installations. The other technique is called Solar Occultation Flux (SOF), which also creates measurements of VOC leaks over large areas.
Before DIAL and SOF, operators of refineries and other industrial installations with the potential for leaks traditionally used emissions factors to estimate VOC losses, supplemented by direct measurements using handheld analysers known as sniffers. Operators still do this, as the same legislation recommending DIAL and SOF also requires operators to use emissions factors, sniffers and another relatively novel technique known as Optical Gas Imaging (OGI).
So, if emissions factors can be so wrong, why do operators still have to use them? Moreover, what do the newer techniques offer to improve to detect and control leaks?
The challenge of the volatile fugitives
Industrial plants can have thousands of potential sources of leaks. Even a medium-sized oil-refinery, for example, could have a total of about 20,000 flanges, valves and pumps, any of which could spring a leak when a seal fails. Storage tanks and effluent-treatment plants are further sources of fugitive emissions.
Fixing leaks is not especially difficult; the real challenge is finding them. All operators use emissions factors for equipment such as flanges, to estimate the amount of leaks per year, based on the premise that a proportion of equipment will start leaking before it is detected. To counter leaks, operators use Leak Detection and Repair (LDAR) programmes, which systematically combine inspection, preventative maintenance, leak detection using sniffers, and repairing equipment that leaks. Sniffers have long underpinned LDAR; these instruments have a probe shaped like a wand, which in turn is linked to a VOC detector. A user then probes around equipment seals to sniff for leaks.
Although sniffers reliably detect leaks, the measurements take time as someone using a sniffer must check every seal of every flange, pump and valve. Additionally, the technique needs calm weather, whilst some items, like tank seals, can be inaccessible. Therefore, in a LDAR programme, some pieces of equipment might be tested annually or less – and a lot can happen in between. On the other hand, this is where DIAL, SOF and OGI are proving to be gamechangers by pointing to the places with leaks.
Dialling up Differential Absorption LIDAR
The first DIAL system used in Gothenburg applied technology that a team from the UK’s National Physical Laboratory (NPL) developed in a joint programme with BP. NPL’s team have since improved the technology and has been performing measurements with DIAL worldwide.
There are few DIAL systems worldwide, yet all have revealed similar patterns of results; for example, expected emissions can be several times higher than estimated values, but not always so. Also teams using DIAL found large leaks from unexpected places, like storage tanks. Furthermore, a few leaks can cause most of the emissions, which could account for errors when using emissions factors.
These discoveries were a revelation for Sweden. The country’s national, environmental regulator now expects all operators of refineries to measure their fugitive emissions at least every three years using long-path techniques such as DIAL, in addition to traditional techniques. Yet despite the advantages of DIAL, industry has not adopted it widely due to perceived drawbacks.
In simple terms, a DIAL system is enormous, expensive and requires highly-trained operators. A decade ago, the refinery-operators’ European trade-body, CONCAWE, was sceptical about the value of DIAL, stating that long-path techniques only provided short-term emission-measurements and could thus lead to large errors in annual inventories. That said, CONCAWE is now more supportive of DIAL, recognising its strengths as a research and investigative tool. Meanwhile, 10 years after the first DIAL investigations in Gothenburg, the strengths and drawbacks of the system inspired work at the city’s technical university to develop a compact, if not alternative technique to DIAL, yet producing similar results.
Enter astrophysics and Solar Occultation Flux (SOF)
The department of Earth and Space Science at Chalmers University in Gothenburg has a team with a long history of using long-path techniques to measuring airborne-chemicals. In the late 1990’s, a team led by Bo Galle and Johan Mellqvist at Chalmers developed a solar-spectroscopic technique which could complement DIAL, or provide an alternative to it, albeit in a smaller, less costly and more flexible package. In simple terms, SOF uses a mobile detector in a van to see how much of the sun’s rays are absorbed by airborne chemicals. Like DIAL, the results are combined with meteorological data to produce graphical images of leaks. Work with SOF has produced similar results to DIAL.
When compared with DIAL, SOF is not as sensitive to some chemicals and has a shorter range of measurement. Yet it is similar in that it needs highly-trained operators and could be viewed as a tool for scientific research rather than routine measurements. Reflecting this, the response so far from operators has been patchy. “Industry, which ultimately pays for the measurements, has defended the emissions-factors model and sniffer methods, whilst regulatory agencies have not taken any strong positions towards measurements SOF either”, Mellqvist from Chalmers comments.
DIAL and SOF can be seen as macro-scale measurements, showing hotspots where close-up monitoring using sniffers can pinpoint leaks; however, this can still be time consuming. Optical Gas Imaging (OGI), on the other hand, has proven an ideal intermediate – and hence its place in new legislation.
Optical Gas Imaging to visualise emissions
OGI instruments are typically housed in a device that resembles a camcorder, and employ a technique known Forward Looking Infrared (FLIR) to detect gases. “We developed the FLIR camera after industry asked for an instrument which could visualise gases”, explains Steve Beynon from FLIR Systems, which makes instruments such as night-vision cameras and thermal imagers. “Industry initially wanted the cameras for hazard investigations, but they have found that OGI systems are beneficial for environmental reasons too, and for preventing losses of products,” adds Beynon.
OGI can be used on its own, yet when used with sniffers, dramatically cuts the time required to find and fix leaks. Mellqvist’s team from Chalmers, for example, uses OGI cameras alongside SOF. “OGI has greatly improved the ability to find leaks,” he comments, adding that: “We use SOF to find hotspot emissions and then use OGI cameras to identify the leaks in the hotspot areas.”
Lastly both Beynon and Mellqvist both emphasise the importance of skilled users for OGI, even if OGI cameras appear simple to use. Standardised, validated methods are essential for users to produce reliable monitoring; to this end, sniffer techniques have long been supported by a United States Environmental Protection Agency procedure known as Method 21, now included within a CEN standard, EN 15446. This in turn is prescribed in the same EC legislation that now specifies sniffers, emissions factors and OGI.
Mellqvist is taking part in an international working-group to develop and validate a supplementary CEN standard for leak detection, to include procedures for DIAL, SOF and OGI. National regulators, NPL, CONCAWE and other industry representatives and are also contributing to this standard. When finished, it will be the final piece of a jigsaw of complementary and synergistic techniques, ranging from DIAL and SOF, through to OGI, sniffer techniques and emissions factors. When combined, they will provide a complete picture of fugitive emissions and more importantly, identify and control them.
Table 1 - Five principal techniques for detecting and quantifying VOC leaks
|Emissions factors for types of equipment that could leak.||Rapid. Based on historical measurements.||High uncertainty, if unexpected and large leaks occur.|
|Sniffer techniques, using handheld instruments.||Semi-quantitative; effective and easy to use, once users have been trained and follow a method rigorously.||Time-consuming and weather-dependent.|
|Optical gas-imaging, using handheld infrared cameras.||Allows operators to rapidly see leaks. Effective and easy to use, if trained users follow a method rigorously.||Not as sensitive to some leaks as sniffing techniques. Weather dependent.|
|Solar Occultation Flux (SOF)||Plant-wide, medium-range technique that creates emissions 2D-profiles and identifies hotspots of leaks.||Requires a medium-sized van and highly-trained operators. Requires meteorological data.|
|DIAL||Plant-wide, far-ranging technique that creates 2D emissions profiles and identifies hotspots of leaks.||Requires a very large vehicle and highly-trained operators. Requires meteorological data.|
Rick Gould MIEMA CEnv works for the Environment Agency, but is writing in a personal capacity as a freelance journalist