Drone-Assisted Fourier-Transform Spectroscopy for Fugitive Emission Sensing

This project is a joint proposal between researchers in Photonics (Reid, Heriot-Watt University) and Robotics (Ramamoorthy, University of Edinburgh). It is a cross-disciplinary collaboration, which is necessary in order to tackle in a new and exciting way the problem of fugitive emissions of methane and volatile hydrocarbons from installations such as refineries, petrochemical plants, carbon-capture and storage facilities and landfill sites. These emissions cost the energy sector up to $5B per year, account for 12% of greenhouse gas emissions and jeopardise worker safety and public health.

Our idea uses mid-infrared laser light to sense the presence of hydrocarbons by looking for characteristic absorptions at wavelengths specific to individual chemical species. Such "gas absorption spectroscopy" is far from new, but we will implement it in a radically different way to conventional approaches. Normally, optical gas detection works by transmitting a single-wavelength through a gas and looking for an intensity change. For fugitive emissions sensing, this is implemented using a technique called DIAL, which shines an intense beam into the air and detects the weak backscattering of this light from particles in the air (Mie scattering). By looking for small differences in the backscattered intensity between two closely-spaced wavelengths, DIAL can sense the presence of one (and only one) chemical species. Its main drawbacks are the weakness of the returned light (after all, air is a very poor reflector!) and its sensitivity only to one chemical species in any given set-up.

The gold standard for lab-based chemical identification is Fourier-transform spectroscopy (FTS), which uses a source similar to a filament light-bulb to explore absorptions over a massive wavelength range all at once. Sadly, such thermal light sources have very poor beam quality, so cannot be transmitted over the long distances appropriate to environmental sensing. In 2004, Heriot-Watt demonstrated that broadband laser light could be used for FTS, combining the wavelength coverage of a thermal source with the beam quality of a laser. This is a game changer for implementing FTS over a long path length as required in environmental sensing, but (for reasons of signal-to-noise) is incompatible with a geometry in which the returned light is very weak. Unmanned aerial vehicle (UAV, or drone) technology has now reached a level of maturity that we can conceive of flying a retroreflector on a UAV to provide a highly efficient means of returning the laser light to a ground-based detector. This concept, which we call DRone-Assisted FTS (DRAFTS), immediately offers improved capabilities over the current state-of-the-art including:

1. Acquisition of concentration and flux maps of multiple chemicals, enabled by using broadband mid-infrared light and allowing correlations to be established and causal effects to be inferred.
2. Sensing with greater range and in diverse atmospheric conditions, since the UAV-mounted retroreflector eliminates the reliance on airborne particles and offers 10,000 times greater efficiency.
3. Deployment in a wider range of scenarios, exploiting the compactness of solid-state lasers, such as using a travelling laser source tracked by the UAV to survey emissions along a road or pipeline.

Working with two key partners -- NPL (a leader in fugitive emissions sensing) and Chromacity (a femtosecond laser manufacturer) -- we aim to evaluate DRAFTS and develop it to a level where we can prove its utility in a simulated fugitive emissions field trial. Our partners are contributing £85K toward the project, and span the supply chain from manufacturer to end-user, thus providing critical opportunities for early commercialization of the DRAFTS concept.

1. SCIENTIFIC IMPACT

Unlike DIAL (the state-of-the-art technique for site-level emissions sensing) the DRAFTS concept uses broadband light. Importantly, this enables the concentrations of multiple chemicals to be measured simultaneously. Scientifically this is a new capability, which for the first time will allow correlations to be observed between different chemicals, in turn enabling hypotheses of causal mechanisms to be formed, scientific explanations to be developed and predictive theories and interventions established. The importance of such correlations is exemplified in the approach taken to understanding the source of California's extra methane, which was identified from the ratio of methane:propane:butane as being from fugitive emissions in the oil and gas sector: Peischl et al. J. Geophys. Res. 118, 4974 (2013). This study also found the rate of fugitive emissions to be 17%, underlining the need for action in this area.

The ability to provide data revealing correlations between different species will have an impact across multiple scientific disciplines, including geophysics, atmospheric science, energy studies and environmental economics. Government will use such data to inform decisions on scientific policy and the direction of tax revenue to solving the problems uncovered by analysing correlations between species in emissions.

Although this project is motivated by exploitation rather than solely by the generation of new knowledge, we expect to share our technical approaches and capabilities with the scientific community through the usual dissemination channels once IP has been appropriately protected. UAV-enabled sensing is only now possible, making it scientifically compelling and highly timely.

2. SOCIETAL IMPACT

The societal cost of uncontrolled emissions is high, and ranges from long-term effects such as climate change to acute safety issues presented by leaks in pipes within petrochemical plants. DRAFTS represents a generic technology for broadband emissions sensing which will be capable of deployment in a wide variety of sectors and scenarios. Its potential impact is therefore very wide, as it could be applied to monitor emissions in sectors as diverse as dairy farming, waste management and energy. Impacts can be expected in areas such as energy policy e.g. government's rate of investment in unconventional fossil fuels like shale gas and the management by multinational energy companies of their assets.

Local communities may also be impacted. For example, DRAFTS could be used to localize the origin of a particular air pollution to a specific site (landfill, traffic, oil refinery etc), providing evidence needed to empower local communities to take legal action against the polluter. By using DRAFTS to trace pollution along a road, the resulting data could be used as a planning tool to understand the origins of traffic pollution on communities.

The project will also impact the early-career researchers it engages, who will benefit from working together in an exciting, cross-disciplinary environment with strong industrial links.

3. ECONOMIC IMPACT

Eliminating fugitive emissions would save the energy sector alone $5B, and DRAFTS technology opens up a more versatile and powerful means of detecting and quantifying these losses. A Californian study found methane leak rates from LA-area oil and gas operations to be 17%, so the economic benefit to the worst affected companies would be massive. In the UK, fugitive emissions (4%) mean a loss to the sector of $20M / annum. With 120 UK oil fields, any investment in DRAFTS sensing technology would recover its cost (£100K estimated) in 1 year, making a strong sales case. Our partner Chromacity can address this market technically, while our partner NPL has many links to UK and international customers for the technology. IP management between our partners and UoE / HWU will be defined before the project begins.