Current and Emerging Laser Sensors for Greenhouse Gas Detection and Monitoring - - Spectroscopy
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Current and Emerging Laser Sensors for Greenhouse Gas Detection and Monitoring


Spectroscopy
Volume 29, Issue 7, pp. 24-33

This article discusses current and emerging tools based on tunable diode laser absorption spectroscopy (TDLAS) for greenhouse gas detection and monitoring, and concludes with some of the technical challenges to overcome in the quest to realize "ubiquitous monitoring."

As scientists, we all know that good science demands good data. In the study of global warming, obtaining good data demands widely deployed, accurate, and reliable sensors for identifying, understanding, and controlling the origins, sources, sinks, and fates of greenhouse gases, especially carbon dioxide (CO2) and methane (CH4). Sensor networks will ideally monitor and map, spatially and temporally, greenhouse gas concentrations with sufficient sensitivity and resolution to distinguish local sources from ambient background and provide fast health and safety danger alerts. These networks will be supplemented by methods for synthesizing, distributing, and using the data provided by the sensors.

This article discusses current and emerging tools based on tunable diode laser absorption spectroscopy (TDLAS) that address these needs, and concludes with some of the technical challenges to overcome in the quest to realize "ubiquitous monitoring."

Sensor Purposes

Methane

Biogenic and anthropogenic methane sources both contribute to greenhouse gas loading. Anthropogenic sources can be controlled and limited if their origins are located. Significant sources are landfills, bovine farms, and the natural gas system, which in the United States includes nearly 500,000 active wells, 300,000 miles of transmission pipelines, and over 1,200,000 miles of distribution pipelines. Natural gas leaks are potential safety risks as well as greenhouse gas sources. Thus, maintaining the system's security and integrity is a legally regulated process of monitoring pipeline flow conditions to recognize abnormal events that may indicate leaks and ruptures, supplemented by scheduled periodic walking, driving, or aerial surveys for locating and repairing leaks.


Table I: US unaccounted for gas, emissions, and significant incidents on natural gas systems (3)
With increasing national emphasis on natural gas as an abundant energy resource, there is increased emphasis on improving leak detection and mitigation. Recent studies show considerable uncertainty in the leakage rate from gas production sites — it ranges from 0.42% (1) to 11% (2). Table I, extracted from a recent study prepared by the office of U.S. Senator Edward Markey, summarizes the national impact of this leakage (3).

High leakage rates have the potential to offset the climate benefits of natural gas over other fossil fuels. Some leakage is believed to be intermittent — but significant — and thus is not detected with routine infrequent leak surveying (4). Continually operating cost-effective leak detector networks, when located judiciously, can help identify these intermittent sources. Continuous monitoring can also provide early warning of abnormal gas concentrations that may indicate a potential safety hazard.

Carbon Dioxide

CO2 is created by combusting fossil fuels, including natural gas, and is participating in climate change (5). To slow or reverse the trend of increasing atmospheric concentration of CO2, geologic carbon sequestration (GCS) is emerging as a strategy for restraining the anthropogenic CO2 flux. GCS involves separating CO2 from combustion effluent, transporting it via pipeline to storage sites, and injecting it at high pressure into storage reservoirs several thousand feet deep. In essence, the process is the opposite of natural gas extraction, but uses very similar pipeline infrastructure. The current 3900 miles of CO2 pipeline in the US is projected to grow to 120,000 miles to support GCS deployment (6), and these pipelines will be subjected to leak detection and repair much like the natural gas pipeline infrastructure. Furthermore, monitoring CO2 at sequestration sites is critical for assuring that the sequestration process is safe and performing its intended purpose — that is, storing CO2 without leakage. Much like natural gas leaks, any leaks from underground sequestration infrastructure, including reservoirs, are likely to be diffusely spread through the ground to emerge from a relatively broad area and be transported by the wind. Thus, economical distributed surface sensor networks with high sensitivity and precision are needed to distinguish leaking CO2 from ambient CO2. The measurement technologies must be sensitive enough to discern CO2 leakage signals that may barely rise above the variations in natural background CO2 concentrations occurring on diurnal to interannual time scales and provide spatial resolution sufficient to localize leak origins.


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