Landfill operators are required to capture and control landfill gas (LFG) as part of the U.S. Environmental Protection Agency’s (EPA’s) New Source Performance Standards (NSPS). These rules are focused on reducing emissions of methane-rich landfill gas from new, modified and reconstructed municipal solid waste (MSW) landfills. The regulations require that landfills perform surface emission monitoring (SEM) to identify potential emission exceedances. Several technologies are used for conducting SEM, including flame ionization detectors (FIDs) and photoionization detectors (PIDs); however, new tunable diode laser absorption spectroscopy (TDLAS) technology can provide several advantages over other options.
Emissions monitoring requirements
EPA’s NSPS regulations require MSW landfills to operate a gas control and collection system (GCCS) to minimize methane emissions. The regulations require that landfills perform quarterly SEM to identify potential emissions greater than 500 parts per million by volume (ppmv). Operators must also ensure that their collection and control system is operating properly. If an exceedance is detected, the landfill must take whatever steps are necessary to correct the issue.
The instruments that can be used for this quarterly SEM are regulated by EPA’s guidance, “Method 21 Determination of Volatile Organic Compound Leaks.” With regards to instrument specifications, the guidance requires that the instrument must:
- Respond to the compounds being processed—in this case, methane.
- Be capable of measuring the leak definition concentration specified in the regulation.
- Have an instrument scale that is to +/- 2.5 percent of the specified concentration.
- Be equipped with an electrically driven pump to ensure that the sample is delivered to the detector at a constant flow rate.
- Be equipped with a probe or probe extension for sampling not to exceed 1.25 inches in outside diameter, with a single opening for admission of the sample.
- Be intrinsically safe for operation in explosive atmospheres.
- Have a response time equal to, or less than, 30 seconds.
Technologies for conducting SEM monitoring
Method 21 states that several technologies are suitable for SEM. Acceptable devices include, but aren’t limited to, those capable of catalytic oxidation, flame ionization, infrared absorption, and photoionization.
Historically, flame ionization detectors (FIDs) are generally accepted as the standard technology for SEM. A FID operates by detecting ions formed during the combustion of organic compounds in a hydrogen flame. The generation of these ions is proportional to the concentration of organic species in the sample gas stream.
While there are advantages to using FIDs, there are also a number of disadvantages. FIDs use an open flame, and there is a risk of flame out. When that happens, FIDs can be difficult to restart. Technicians must carry around bottled hydrogen, which can be difficult to obtain and transport. Hydrogen is highly flammable and cannot be shipped to locations like standard calibration gases can, so it must be obtained locally.
FIDs can also be heavy, with some weighing as much as 12 pounds. While that may not seem like a lot, carrying these around a landfill all day, even in a backpack, can create strain and fatigue for operators.
Many detection methods require the use of two separate devices—one to take the sample and one to save the data and the GPS coordinates, which must be reported along with the sample. Among other considerations, this means technicians must track the status of two batteries to be sure each is charged and ready to use.
TDLAS technology offers versatility and convenience to help landfill operators meet the EPA’s SEM requirements without the drawbacks of other technologies.
Finally, FIDs can read a wide range of hydrocarbons, but they can be susceptible to a cross-gas effect where you get false readings due to, or influenced by, the presence of other gases or hydrocarbons.
Another technology that can be used for SEM is the photoionization detector (PID), which uses high-energy photons in the ultraviolet (UV) range to break molecules into positively charged ions. As compounds enter the detector, they are bombarded by high-energy UV photons and are ionized when they absorb the UV light, resulting in ejection of electrons and the formation of positively charged ions. The ions produce an electric current, which is the signal output of the detector. The greater the concentration of the component, the more ions are produced, and the greater the current. The current is amplified and displayed on an ammeter or digital concentration display.
PIDs can detect multiple gases and are commonly used to detect volatile organic compounds (VOCs). They have a rapid response time but require frequent cleaning. In addition, the UV lamps wear out and require replacement.
A more recent SEM technology uses TDLAS, a combination of laser absorption spectrometry and applied electronic signals. An electronic signal is applied to increase the accuracy of the laser tuning range, which means the device can focus on the methane only within the spectrum of the sample. The device filters out any other VOCs so that they will not register and cannot influence or affect the methane concentration readings.
A true laser-based device offers many features over other technologies. First is accuracy, as these can detect readings down to 0.5 ppm. Additionally, no flame is required, which is a significant benefit for sampling in a potentially explosive environment. No external gas bottle is required for operation, and the laser technology also eliminates the risk of flame out, so the user does not lose time stopping and relighting. By including integrated GPS and Bluetooth, a well-designed TDLAS instrument can also eliminate the need for a secondary device for data storage and GPS. With proper internal storage, the detector can hold up to 480 hours of scan data, or roughly 3 months of data, before it must be downloaded. TDLAS design also allows for a weight that is less than half that of other SEM monitoring options, which is easier for the operator.
Both TDLAS instruments and software can help the user remain compliant with environmental regulations. Readings can be saved to a computer, and tools such as user-selectable reporting functions, accessible site maps and scan paths, and customizable indicator points based on the scan are available. These software programs can also format the scan data and autofill industry-standard background reports to make the job easier for operators.
An effort to improve SEM results
Efforts to capture, control and measure excess landfill gases are critically important given current regulations governing landfill emissions. And while SEM is required by the EPA, not all technologies approved to perform SEM are equal. New laser-based technology can help operators meet and exceed EPA Method 21 requirements for quarterly SEM monitoring. This TDLAS technology offers versatility and convenience to help landfill operators meet the EPA’s SEM requirements without experiencing the drawbacks of other technologies in terms of safety, efficiency and ease of use.