Mobile laser lab provides comprehensive emissions monitoring where it’s most needed

Mobile DIAL, or differential absorption light detection and ranging (LiDAR) labs, use lasers to scan any area and accurately monitor the source and volume of emissions – whether from manufacturing, chemical and petroleum processing or power generation – to help reduce their environmental and commercial impact.

Measuring emissions

“You can’t manage what you can’t measure,” is a phrase often used, and never is it more true than for measuring the emissions from atmospheric pollutants. Accurate measurement is increasingly important if we are to meet corporate, national and international emissions targets such as the EC directive for integrated pollution prevention and control (IPPC) and to reduce the environmental impact of industry.

Not only are emissions harmful to the environment, but they are also often damaging to companies’ bottom lines as they waste valuable gas.

Responsible businesses are keen to detect greenhouse gas leaks to reduce their impact on global warming, comply with regulations, support company environmental reporting and improve their efficiency. As such there is a growing demand for facilities that can accurately identify and characterise leaks.

There are several techniques used to measure emissions. Some technique involve the measurement of several points downwind from the source, to draw conclusions about what sort of plume would have given rise to that effect. In order to use these, however, you need to know the source of the emissions to begin with.

Others use tracer, released at the source and measure other points to calculate the movement and dispersion of the emission plume. This works if you can guarantee that what happens to the tracer happens to the methane, but it means you need to know a lot about your emissions in order to put your tracer in the right place and quantity.

A third method is the use of mirrors and lasers to measure the concentration of gas in between. This provides a concentration path average, but modelling is needed to build up a picture of the plume and the system is not flexible – if the wind changes then you have to wait until it returns to the direction that your mirrors are stationed in.

All of these techniques require existing knowledge about the emission plume such as source and size, and often, particularly with fugitive emissions or leaks, this will not be available. They also often use modelling to make estimations of what is happening, instead of providing an actual picture of what is happening at a given time.

DIAL

Using optical radar, the location, volume and concentration of emissions can be characterised, with only prior knowledge of the emission species (i.e. the gas to be measured) required.

Mobile emissions monitoring systems have been developed using differential absorption LiDAR (DIAL), a sophisticated laser-based remote sensing tool. Just like radar, differential absorption LiDAR measures the reflection of transmitted signals. DIAL fires pulses of light at different wavelengths through the gas plume. A small fraction of this light is scattered back and collected with a telescope and a very sensitive detector, allowing the concentration and location of the gas in the atmosphere to be measured. This enables the size, volume, direction and spatial distribution of the plume to be determined.

Existing data on the absorption of the laser light by different target gases previously carried out in spectroscopic facilities can then be used to convert the raw DIAL signal into concentration measurements. Combining these concentration measurements with measurements of the wind speed and direction enables the emission rate in kg per hour of the target gas to be determined.

DIAL can measure in the infrared, visible and ultra-violet spectral regions, characterising species including sulphur dioxide, nitrogen dioxide, nitric oxide, ozone, benzene, toluene, xylene and higher aromatics, alkanes, alkenes, petroleum and dieselvapours, mercury, hydrogen chloride, nitrous oxide, hydrogen fluoride and hydrogen sulphide.

The benefit of using a remote optical technique like DIAL is that it can measure total site emissions, rather than just gases at a single point. It can also monitor diffuse sources such as landfill or lagoons. The results generated from optical techniques are therefore better to use for measuring regulatory compliance for industrial sites, evaluating impact and for early warning of any issues.

DIAL is particularly useful in measuring and assessing fugitive emissions, including leaks from processing plant, storage tanks and wastewater treatment, or emissions from area sources such as landfill. Fugitive emissions are potentially damaging to the environment and yet are rarely identified, let alone measured and addressed. Studies in the US and Canada showed that measured total-site emissions from refineries can be up to ten times higher than calculated, due to fugitive emissions.

Mobile DIAL systems are self-contained mobile laboratories, which are able to track and quantify plumes emitted from area emissions remotely, at ranges of up to 1km. They can provide measurements incredibly quickly and be shipped, or driven, anywhere monitoring is needed.

There are, of course, benefits to having more permanent systems in place to track emissions over time, but DIAL allows a comprehensive survey of the emission plume to be completed. It is also very flexible and can carry on measuring in almost any conditions. If the wind changes, for example, the measurement beam can simply be scanned in a different direction or the whole system can be moved very quickly, eliminating long wait times. Additionally, the light source can easily be tuned to different wavelengths to characterise plumes of different gases in one visit.

The measurements produced by DIAL are traceable to primary standards of gas concentration and are free from interference and contamination.

The DIAL technique is particularly suited for use in the following applications: • Remote measurements of inaccessible, hazardous or elevated areas • Wide area surveys of diffuse sources, such as methane from landfill sites • Measurement of total industrial site emissions, including flares and tanks at petrochemical plants • Boundary fence monitoring • Identification and quantification of leaks and fugitive emissions at conventional and unconventional oil and gas sites • Plume tracking and source identification from complex industrial plants

DIAL in operation

Numerous measurements of industrial emissions have been carried out using DIAL: from large scale studies of volatile organic components (VOCs) at refineries, to measurements of methane losses from landfill sites. It has been used in many countries, including the Netherlands, Belgium, Norway, Germany, Spain, Greece, the UK, USA, France, Canada and Ireland.

Source identification

One on assignment, the DIAL facility was used to measure controlled and fugitive emissions of VOCs from all areas of a natural gas terminal. Because DIAL is a remote sensing technique, there was no requirement to enter any hazardous areas. The measurements focused on the emissions from the storage tanks, vent stacks and flares of the terminal across a five day campaign and were used to determine the emissions in kilograms per hour of benzene and methane. In addition, headspace analysis was carried out to determine the concentrations of different VOCs in the tank vapour spaces.

The DIAL was deployed around the perimeter of the site and within the site on plant access roads to measure fluxes from each site area: storage tanks, process area, compressors, ground flares and boilers to name a few. DIAL performed vertical scans of the concentration of these gases, downwind of the targeted sources. These measurements provided a complete two-dimensional (2D) map of the pollutant concentrations. In addition, the wind vector was monitored using the meteorological mast mounted on the DIAL system and a separate 12m mast deployed in an open area of the site for the duration of the monitoring campaign. Simultaneous sampling of the air mass being monitored was carried out. The sampled air was subsequently analysed using gas chromatography to provide a breakdown of the hydrocarbons in the gas.

In the above example the DIAL system used two modes, one mode to measure benzene and the other to measure methane. The system was therefore tuned in turn to the specific wavelengths, which enabled the scattering of each gas species to calculate concentration.

A total of 54 measurement scans were carried out at the natural gas terminal during this visit. Each scan measured the concentration distribution of VOCs, either benzene or methane. A number of measurements were made of the emissions from the tankage areas on site. These were made under different wind conditions and using angles across the site, which enabled the emissions from individual tanks to be determined.

A single tank was observed to be the largest single emission source and it accounted for almost half the emission flux from the tanks on site. By scanning the DIAL in different geometries, different sources can be identified and quantified.

Another recent project was with the local Environmental Protection Agency (DCMR) at the Port of Rotterdam, the Netherlands, which found an increased concentration of the carcinogen benzene in the air. As with the natural gas terminal project above, the DIAL system was used to create a report identifying the source of the leaks. Based on this report and expert advice, the local agency identified and brought the source of the leaks under control, and has subsequently purchased a specialised camera with which they can make the invisible benzene visible, so that future leaks can be tracked on an ongoing basis.

Wide area survey

The combination of wide area coverage and range-resolved measurements means that the DIAL method is well suited to carrying out detailed studies of emissions from large area sources. Since the DIAL method directly maps the atmospheric distribution of the emissions, the emitted flux can be calculated without assumptions about source distribution and plume behaviour or reference to dispersion models. This removes one major source of uncertainty from flux results and means that DIAL can be used to measure emissions when topography or weather conditions would lead to unreliable dispersion modelling.

Another benefit of the DIAL method in this type of application is that by being self-contained and mobile it allows the safe measurement of emissions from hazardous or difficult to access sites.

A good example of area methane sources is the emission of landfill gas from waste management sites. Landfill gas results from the bio-degradation of the organic fraction of landfilled waste, and typically contains between 40% – 60% methane. The distribution of emissions across any given area varies significantly with the site activity at the time of measurement, the content of the waste being landfilled and the route to surface of the emitted methane, which can involve diffusion through the top layers, escape through faults in any gas control system or migration through geological features.

In a recent study in France, a comparison was made of five different methods for measuring fugitive methane emissions from a landfill site, including those techniques listed at the beginning of the article. As an example, in two days of measurement the DIAL system proved able to quantify the emissions from different areas of the landfill and identify and map the localised emission sources around the site. The flux measurements showed that, as expected, the active cell (where landfill was being actively tipped) had the highest emission levels (30 ± 7kg/hr) with the closed cell emissions varying between 1.5 and 10kg/hr depending on the age and content of the closed cell.

The localised emissions from the gas engine were measured specifically and showed a low flux rate of around 0.8kg/hr. Given the size of the site, the emission rate corresponded to approximately 15kg/hr/hectare for the active area on this site.

The results of this study confirmed the ability of DIAL to provide methane flux measurements and 2D visualisations of the emission distributions, without requiring specific site access or configuration, unlike other methods. The conclusion was that, although the DIAL system was bigger and more complex than the other methods, it had greater potential to be deployed at any landfill site and provide an accurate area-by-area analysis of the site emissions.

Another recent landfill study investigated the methane emissions from two adjacent landfill sites in the USA. The emissions from each area of the site were mapped and quantified over the six day measurement campaign which also included half a day measuring a controlled methane release. As with the previous study, the active tipping areas of the sites were the source of the majority of the emissions, although in this case it was noted that the emission levels from the active areas on both sites were significantly higher during periods of active tipping.

The area-weighted emission rates from the active areas were approximately 40kg/hr/hertare, 55kg/hr/hectare during the tipping periods, and approximately 15kg/hr/hectare and 14kg/hr/hectare during the quiet periods. Again, these measurements have shown the capability of DIAL to quantify the methane emissions from complex large area sources in a short space of time.

DIAL has recently been used to monitor emissions from landfill sites in the UK by Defra and DECC.

Published: 19th Aug 2014 in AWE International