Elsevier

Waste Management

Volume 32, Issue 2, February 2012, Pages 297-304
Waste Management

Photoacoustic infrared spectroscopy for conducting gas tracer tests and measuring water saturations in landfills

https://doi.org/10.1016/j.wasman.2011.09.016Get rights and content

Abstract

Gas tracer tests can be used to determine gas flow patterns within landfills, quantify volatile contaminant residence time, and measure water within refuse. While gas chromatography (GC) has been traditionally used to analyze gas tracers in refuse, photoacoustic spectroscopy (PAS) might allow real-time measurements with reduced personnel costs and greater mobility and ease of use. Laboratory and field experiments were conducted to evaluate the efficacy of PAS for conducting gas tracer tests in landfills. Two tracer gases, difluoromethane (DFM) and sulfur hexafluoride (SF6), were measured with a commercial PAS instrument. Relative measurement errors were invariant with tracer concentration but influenced by background gas: errors were 1–3% in landfill gas but 4–5% in air. Two partitioning gas tracer tests were conducted in an aerobic landfill, and limits of detection (LODs) were 3–4 times larger for DFM with PAS versus GC due to temporal changes in background signals. While higher LODs can be compensated by injecting larger tracer mass, changes in background signals increased the uncertainty in measured water saturations by up to 25% over comparable GC methods. PAS has distinct advantages over GC with respect to personnel costs and ease of use, although for field applications GC analyses of select samples are recommended to quantify instrument interferences.

Highlights

► Photoacoustic infrared spectroscopy tested for measuring tracer gas in landfills. ► Measurement errors for tracer gases were 1–3% in landfill gas. ► Background signals from landfill gas result in elevated limits of detection. ► Technique is much less expensive and easier to use than GC.

Introduction

Reducing greenhouse gas emissions from landfills is aided by a better understanding of gas flow patterns in refuse, particularly as they are modified by landfill gas collection systems (Borjesson et al., 2007). Gas flow patterns might be ascertained by conducting gas tracer tests, which have been used to assess gas transport in the vadose zone (Christophersen et al., 2005). Similar measurements in landfills involve the injection of tracers within refuse and measurement at one or more gas collection wells. In addition to providing information on gas transport, such data could also be used to determine the residence time distribution of gas generated at different locations within the waste, information needed for developing models for the fate and transport of pollutants within solid waste landfills (Lowry et al., 2008). For both purposes, measurement systems are needed that are simple to use and sufficiently accurate for measuring gas tracers in multicomponent landfill gas (LFG).

Gas tracer tests might also be useful for measuring the distribution of liquid in landfills. Moisture in refuse is critical for waste decomposition (Reinhart and Townsend, 1998), and therefore frequent measurements of liquid within landfills are needed to know how much water to add and where to add it. The partitioning gas tracer test (PGTT) used for measuring water and nonaqueous phase liquids in the vadose zone has been evaluated for measuring moisture in landfills (Han et al., 2007). The PGTT is based on the different affinities of two tracer gases for water – a partitioning tracer that is absorbed by water, and a conservative tracer. Depending on the amount of water in the zone swept by the gas tracers, transport of the partitioning tracer is retarded with respect to the conservative tracer. The degree of retardation is used to determine the amount of water present in the system, which is an integrated value over the contact area of the tracer gases.

The PGTT has particular advantages when applied to highly biologically-active systems such as landfills. Extracting waste samples for gravimetric measurements are generally infrequent since it requires mobilizing a drill rig and may expose workers to solid wastes. Other traditional methods for measuring soil water content (e.g., neutron probe, time domain reflectrometry) require that sensors be calibrated using samples extracted from the field. These methods also require the pore structure to be temporally invariant (Imhoff et al., 2007), a condition that is not satisfied for long periods in landfills. Thus, the PGTT is a promising nondestructive, in situ method for measuring water saturation (Sw), defined as the fraction of the pore space filled with water.

The challenge of applying the PGTT, though, resides in collecting and analyzing gas samples, which traditionally has required use of field-portable gas chromatographs and associated consumables (GC-Field) (Han et al., 2007), or gas sampling systems in the field, ideally automated, and measurement of collected samples in the laboratory (GC-Lab) (Han et al., 2006, Keller and Brusseau, 2003). Simpler systems requiring less effort and time for gas sampling and analysis would permit a wider application of PGTTs in landfills.

Photoacoustic infrared spectroscopy (PAS) may be a viable on-site measurement technology for analyzing gas tracer data from tests designed to quantify gas flow patterns or PGTTs. PAS operates based on the conversion of light energy into sound energy, and the basic principles are described elsewhere (Harren et al., 2000). Briefly, infrared (IR) light from lasers is modulated at a particular frequency (Lindley et al., 2007), or broadband IR sources are optically filtered to a prescribed wavelength (Fonsen et al., 2009), before entering a sampling chamber. Gaseous molecules within the chamber absorb electromagnetic radiation at characteristic wavelengths defined by their molecular structure. After excitation to a higher quantum state, the gaseous molecules depopulate to lower states causing an increase in pressure inside the chamber, and the resulting sound energy is detected with a sensitive microphone. While the most sensitive measurements are obtained using lasers, sometimes ∼10 ppb levels (Lindley et al., 2007), PAS utilizing broadband sources are noted for their reliability and cost effectiveness (Harren et al., 2000). A particular advantage of PAS over gas chromatography (GC) methods is sampling speed: measurements of trace gases can be made in seconds (Fonsen et al., 2009), while online GC measurements typically take on the order of minutes or longer, depending on the chromatographic separation required for other gas components. Depending on the particular instrument design, detection limits for PAS may be poorer than, equivalent to, or exceed that of GC methods. For example, a PAS using a tunable diode laser was used to detect methane (CH4) at 23 ppb, while a GC-FID equipped with a specially designed microtrap had a detection limit of 28 ppb (Thammakheta et al., 2005). In contrast, a commercial PAS unit with an optical broadband source is reported to have a detection limit for CH4 of 0.1 ppm (INNOVA Model 1312, Lumasense Technologies, Denmark).

For multicomponent analysis of a gas mixture, a laser may be tuned to multiple laser lines that are selected for excitation and measurement of the different gases (Harren et al., 2000). For a broadband light source, multiple optical filters can be used to select wavelengths appropriate for distinguishing between the gases. However, since gas molecules absorb light energy over a wide range of the IR spectrum, interferences between gas components might be inevitable, resulting in erroneous measurements (Harren et al., 2000). Therefore, sometimes a cross interference calibration is necessary to compensate for the effect of interfering substances. Assuming the photoacoustic response for each gas in a mixture at each laser line or optical filter wavelength is linear and the absorption coefficients are known, a series of linear equations are solved to determine the concentrations of the gases in a multicomponent mixture (Harren et al., 2000).

This cross interference calibration approach is used in a commercial instrument (INNOVA Model 1312, Lumasense Technologies, Denmark) that employs a broadband light source and optical filters. CH4 fluxes from composting operations were measured with this instrument and compared with online GC-FID (Osada and Fukumoto, 2001). Total CH4 emissions differed by less than 5%, which may have been due to high background concentrations of carbon dioxide (CO2) (Osada and Fukumoto, 2001) or to the absorption of light by unknown volatile organic compounds (VOCs) not quantified with the PAS but separated from CH4 for GC-FID. Cross interferences were also suspected when a PAS instrument equipped with a CO2 laser was used to detect ethene in the atmosphere (Kuster et al., 2005). The PAS had a positive offset of 330 ± 140 ppt relative to the GC-FID, which was attributed to either the baseline signal for the instrument or to interference from other VOCs not accounted for in the PAS calibration. The effect of this offset was to increase the detection limit of ethene to ∼1 ppb for the PAS.

While PAS instruments hold promise for rapid, online quantification of gas tracers in LFG, depending on the light source and instrument design interferences from unknown constituents in LFG may be problematic. LFG contains numerous IR-absorbing gas components although the concentrations of most are much lower than that of CH4, CO2, and water (Allen et al., 1997, Ecklund et al., 1998). In this study laboratory and field experiments were conducted to assess the accuracy of a commercial PAS instrument that utilizes a broad band light source and optical filters for measuring tracer gases in LFG. A modeling analysis was used to predict the impact of individual LFG constituents on the performance of this PAS instrument. The results from these experiments and modeling analysis were used to assess the utility of PAS for conducting gas tracer tests in landfills.

Section snippets

PAS instrument

The PAS instrument used was an INNOVA Model 1412 Photoacoustic Field Gas-Monitor (LumaSense Technologies, Denmark) that can be fitted with six optical filters, one of which must be selected to quantify water vapor, which absorbs IR light at nearly all wavelengths. The selection of the five remaining optical filters is dependent on the gases of interest and any anticipated interfering gases. For PGTTs, sulfur hexafluoride (SF6) was selected as the conservative tracer. If an alternative gas with

Accuracy of PAS instrument

The accuracy of the PAS instrument was evaluated using laboratory measurements of DFM and SF6 in air and LFG and the results are shown in Fig. 2. With air as a background gas, the signals recorded for DFM and SF6 with no tracer gases present were 0.043 and 0.011 ppm, respectively. Subtracting these background signals from DFM and SF6 signals, the mean measurement errors averaged over all samples across the entire concentration range were 4.7 ± 1.3 CI% (CI = 95% confidence interval) and 4.2 ± 0.8 CI%

Discussion and conclusions

As a tool for real-time measurements of gas tracers in landfills, the accuracy of a commercial PAS instrument using a broad band light source and optical filters was evaluated. The performance of the PAS instrument for measuring two tracers, DFM and SF6, in LFG was examined by conducting laboratory and field experiments and modeling analysis.

The accuracy of PAS measurement of DFM and SF6 was evaluated in the laboratory. Mean errors were 4.7 ± 1.3 CI% and 4.2 ± 0.8 CI% for DFM and SF6 measurements

Acknowledgements

This work was supported by the US Department of Energy under Cooperative Agreement DE-FC26-05NT42432 and the California Energy Commission.

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