Lightweight device filters out other VOCs so they do not affect methane concentration readings

By Dustin Pickering, Environmental Products Application Specialist, QED Environmental Systems, Inc.

Landfill operators are required to capture and control landfill gas (LFG) as part of the U.S. Environmental Protection Agency'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). New tunable diode laser absorption spectroscopy (TDLAS) technology has a number of real advantages over other options.

Overview of landfill surface 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 (40 CFR part 60 subpart XXX), require that landfills perform quarterly SEM to identify potential emissions greater than 500 parts per million by volume (ppmv). They 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 Method 21 - Determination of Volatile Organic Compound Leaks. With regards to instrument specifications, Method 21 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.

Method 21 governs the instrument specifications, and calibration and performance guidelines for equipment used for surface emissions monitoring. Instruments selected must meet these guidelines.

Technologies suitable for conducting the required SEM monitoring

Method 21 states that several technologies are suitable for SEM. Acceptable devices include but aren't limited to catalytic oxidation, flame ionization, infrared absorption, and photoionization.

Historically, flame ionization detectors (FIDs) are generally accepted as the standard technology for SEM. An FID operates by detecting ions formed during 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. [1]

While there are advantages to using FIDs, there are also number of disadvantages. FIDs use an open flame and there is a risk of flame out. When that happens, they 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. The devices can also be heavy, with some FIDs weighing as much as 12 pounds. While that may not seem like a lot, carrying it 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 stake the sample and one to save the data and the GPS coordinates, which must be reported along with the sample. This means technicians must track the status of two batteries to be sure each is charged and ready to use.

Finally, FIDs can read a wide range of hydrocarbons, but they can be susceptible to a cross-gas effect - that is, 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. [2]

PIDs can detect multiple gases, and are commonly used to detect volatile organic compounds (VOCs). The 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 tunable diode laser absorption spectroscopy (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.

One example is the LANDTEC SEM5000 methane (CH4) detector from QED, a laser-based device that is accurate down to 0.5 ppm. No flame is required, a huge benefit for sampling in a potentially explosive environment. No external gas bottle is required for operation; the laser technology also eliminates the risk of flame out, so the user does not lose time stopping and relighting. It comes with an integral GPS and Bluetooth that eliminates the need for a secondary device for data storage and GPS. The detector holds up to 480 hours of scan data - almost 3 months! - before it must be downloaded. It also features an ergonomically designed package that weighs only 3.5 pounds, less than half that of other SEM monitoring options.

The detector also features a "hot swap" removable and rechargeable battery, which means users charge the battery, not the instrument. When the charge from one battery has been depleted, users can simply swap to a fully charged battery and keep working. Each battery provides about 10 hours of operation on a single charge. Readings can be saved to your computer using the proprietary SEMsoft software package, which comes with multiple reporting functions and choices, easy upload for site maps and scan paths, and customizable indicator points based on the scan. The software formats the scan data into the standard .csv report, and auto-fills industry standard background reports.

Conclusion: Improve SEM results with new laser technology

Efforts to capture, control, and measure excess landfill gases are critically important, given current regulations around landfill emissions. Surface emission monitoring is required by the EPA, though not all technologies approved to perform SEM are equal. New laser-based technology, like the SEM5000, meets and exceeds EPA Method 21 requirements for quarterly SEM monitoring. The device's versatility and convenience helps landfills to meet the EPA's SEM requirements without experiencing some of the drawbacks of other technologies in terms of safety, efficiency, and ease of use.

References

[1] Flame ionization detector, Wikipedia, https://en.wikipedia.org/wiki/Flame_ionization_detector, retrieved 6/5/18.
[2] Photoionization detector, Wikipedia, https://en.wikipedia.org/wiki/Photoionization_detector, retrieved 6/5/18.