Waste Management xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
Quantifying capture efficiency of gas collection wells with gas tracers Ramin Yazdani a,b,⇑, Paul Imhoff c, Byunghyun Han c, Changen Mei c,d, Don Augenstein e a
Air Quality Research Center, University of California, Davis, CA 95616, USA Yolo County Division of Integrated Waste Management, Woodland, CA 95776, USA c Department of Civil and Environmental Engineering, University of Delaware, 344A DuPont Hall, Newark, DE 19716, USA d Geoscience and Environmental Engineering, Southwest Jiaotong University, Chengdu, Sichuan 610031, China e Institute for Environmental Management (IEM), Inc, Palo Alto, CA 94306, USA b
a r t i c l e
i n f o
Article history: Received 8 April 2015 Revised 20 June 2015 Accepted 22 June 2015 Available online xxxx Keywords: Methane emissions Field measurements Well radius of influence
a b s t r a c t A new in situ method for directly measuring the gas collection efficiency in the region around a gas extraction well was developed. Thirteen tests were conducted by injecting a small volume of gas tracer sequentially at different locations in the landfill cell, and the gas tracer mass collected from each test was used to assess the collection efficiency at each injection point. For 11 tests the gas collection was excellent, always exceeding 70% with seven tests showing a collection efficiency exceeding 90%. For one test the gas collection efficiency was 8 ± 6%. Here, the poor efficiency was associated with a water-laden refuse or remnant daily cover soil located between the point of tracer injection and the extraction well. The utility of in situ gas tracer tests for quantifying landfill gas capture at particular locations within a landfill cell was demonstrated. While there are certainly limitations to this technology, this method may be a valuable tool to help answer questions related to landfill gas collection efficiency and gas flow within landfills. Quantitative data from tracer tests may help assess the utility and cost-effectiveness of alternative cover systems, well designs and landfill gas collection management practices. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Methane (CH4) is an important contributor to global warming with a radiative forcing estimated to be close to 20% that of carbon dioxide (CO2) over the last two decades (IPCC, 2013). If the change in carbon storage due to climate change is taken into account the global warming potential of CH4 has been determined to be 34 times more than CO2 (IPCC, 2013). In 2012, landfill CH4 emissions represented the third largest source of CH4 emissions in the United States, behind natural gas systems and enteric fermentation (US EPA, 2014a,b). Budgets are needed to inventory landfill CH4 emissions. These can be expressed as production = recovery in gas wells + oxidation in cover soils + surface emissions + subsurface migration + D storage (Bogner and Spokas, 1993; Spokas et al., 2006), although the migration and storage terms are sometimes neglected (Börjesson et al., 2009). In this case, landfill CH4 collection efficiency is computed as recovery/production, where recovery is measured directly from the sum of all gas collection wells, and production is the sum
⇑ Corresponding author at: Air Quality Research Center, University of California, Davis, CA 95616, USA. E-mail address: [email protected] (R. Yazdani).
of recovery + oxidation + emissions (Börjesson et al., 2009) and may include + migration + D storage (Spokas et al., 2006). While this procedure has been used to evaluate the CH4 collection efficiency of an entire landfill, it is an expensive indirect measurement that because of cost is usually performed infrequently. In addition, this approach cannot be easily used to assess the efficiency of particular gas collection well designs, which may be installed at only a few locations. If instead the efficiency of landfill gas collection is assumed, CH4 production may be estimated from recovery data alone. This approach was recently applied to develop and improve U.S. EPA’s Landfill Gas Emissions Model (LandGEM) (Wang et al., 2013, 2015). In this case, the efficiency of landfill gas collection was based on expert judgment, using information about waste age, well density, and cover type on the landfill surface (Wang et al., 2013). Unfortunately, there can be significant uncertainty in estimates of landfill gas collection efficiency (Wang et al., 2015). Direct measurement of landfill gas collection efficiency may improve such modeling efforts. In addition, such measurements may help to enhance landfill gas collection by improving operation of landfill gas collection systems and design of gas collection wells. In two previous studies (Han et al., 2007; Yazdani et al., 2010) partitioning gas tracer tests were successfully performed to
http://dx.doi.org/10.1016/j.wasman.2015.06.032 0956-053X/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Yazdani, R., et al. Quantifying capture efficiency of gas collection wells with gas tracers. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.06.032
2
R. Yazdani et al. / Waste Management xxx (2015) xxx–xxx
measure waste moisture content in a full-scale bioreactor landfill. While the collection efficiencies of the injected gas tracers were computed, tests were too few to assess the variation of collection efficiency around the extraction well, the repeatability of measurements, and the relationship between collection efficiency and gas suction at the point of tracer injection. In this work in situ gas tracers were conducted that address these limitations. In these tests, a conservative tracer gas is injected at multiple locations near a gas collection well where gas pressure is measured. Using the known mass of tracer gas injected and the mass recovered from the gas collection well, the collection efficiency is determined. The collection efficiency is measured for specific locations surrounding a gas collection well, providing a picture of how collection efficiency varies in space and perhaps time for repeated tracer tests, due to changes in atmospheric conditions or landfill operations. As distinct from other approaches, employing gas tracers to determine landfill gas collection efficiency is a direct measurement. However, because this approach requires tracer injection at individual points in a landfill, the method determines the collection efficiency for these points only; multiple points are needed to assess collection efficiency surrounding a well. The traditional definition of CH4 collection efficiency is CH4 gas recovery/production. At any point in time it is likely production varies spatially within waste in an unknown pattern associated with waste composition and moisture condition. The average collection efficiency of tracers injected at multiple points within a waste mass may not be identical to CH4 gas recovery/production in the sampled region. Landfill gas may be generated in nonuniform patterns surrounding gas collection wells that cannot be precisely matched with tracer data: it is not possible to weight the collection efficiency of tracer gases with the corresponding production of CH4 at each tracer injection point, since CH4 production at these points is unknown. Thus, the gas tracer technology proposed here provides gas collection efficiencies that when integrated in space surrounding a well may not be identical to overall CH4 collection efficiency in the sampled region. Nevertheless, gas collection efficiencies from tracer tests at multiple points in a landfill cell provide a picture of spatially varying collection efficiency, where it is good and where it is poor, which should correlate with regions of good and poor CH4 collection efficiency. The maximum radius of influence (ROI) of a vertical gas collection well can be defined according to a methodology established by the US EPA (US EPA, 1996). Briefly, the ROI is the furthermost distance from a gas collection well where in situ gas pressures exhibit measurable suction from well vacuum. The ROI is the average distance determined from gas probes located along three radial arms 120° apart and at depth equal to the top of the screened well. Because precision of gas pressure measurement varies with instrument, the ROI is not well-defined and is generally smaller for less precise pressure measurements (Walter, 2003). Despite this limitation, we are unaware of data quantifying the collection efficiency for points within the ROI. Implicit in empirical well test methods for estimating landfill gas production (Walter, 2003) is that collection efficiency approaches 100% within the ROI. In this study the gas tracer technology outlined above was developed and tested for measuring the landfill gas capture efficiency at points surrounding a vertical gas collection well. This technology could be used to quantify the benefits of alternative landfill gas well designs, to track the efficiency of an existing collection system as a landfill cell moves from active to intermediate to final cover, or to evaluate various operational changes, e.g., modifications to the gas collection system to mitigate the effects of barometric pressure changes. Gas collection efficiency was assessed at points within the ROI of one well to illustrate how collection efficiency might vary within this region.
2. Materials and methods The focus of this work was the testing and development of an in situ gas tracer technique and included the following tasks: (1) design, construction, and installation of field testing equipment; (2) completion of multiple in situ tracer tests under varying pumping conditions and climatic settings; (3) measurement of gas pressure fields for gas tracer tests; (4) measurement of moisture conditions in cover soil; and (5) analysis of in situ gas tracer tests to determine gas well collection efficiency. The procedures and methodologies for Tasks 1–4 are described below. The procedures for analyzing gas tracer tests for Task 5 are discussed in detail in other publications (Han et al. 2006, 2007). 2.1. Field construction and instrument installation Field tests of the in situ gas tracer technology were performed at the Yolo County Central Landfill in Woodland, California. The test area was within a 6-hectare landfill cell (average waste depth of 9 m) that received 400,000 tonnes of municipal solid waste between 2003 and 2006. The average gas flow rate from this landfill cell from 10 gas wells during the field tests was 180 SCMH. During the field tests none of the gas wells within the 6-hectare area, other than test well D23 described below, were adjusted. The landfill cell was covered with an intermediate soil cover consisting of 50–60 cm of compacted clay soil (Capay silty clay) taken from a borrow site adjacent to the landfill. In some regions, an additional 15–18 cm of compost overlaid the cover soil. In situ gas tracer tests were conducted in the vicinity of vertical gas collection well D23, which was 9.1 m in depth and consists of a 10.2 cm diameter Schedule 80 PVC pipe perforated at the bottom 4.6 m. A schematic of the well design is shown in Fig. 1. The closest vertical gas well was 31 m away to the north. To measure the volumetric flow rate of landfill gas collected from gas well D23 during the tracer tests, a Roots Meter Series B3 5M175 flow meter (Dresser Inc., Houston, TX) was used with a measurement error
Fig. 1. Schematic of vertical gas collection well D23 used in the field. Not to scale.
Please cite this article in press as: Yazdani, R., et al. Quantifying capture efficiency of gas collection wells with gas tracers. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.06.032
R. Yazdani et al. / Waste Management xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Waste Management journal homepage: www.elsevier.com/locate/wasman
Quantifying capture efficiency of gas collection wells with gas tracers Ramin Yazdani a,b,⇑, Paul Imhoff c, Byunghyun Han c, Changen Mei c,d, Don Augenstein e a
Air Quality Research Center, University of California, Davis, CA 95616, USA Yolo County Division of Integrated Waste Management, Woodland, CA 95776, USA c Department of Civil and Environmental Engineering, University of Delaware, 344A DuPont Hall, Newark, DE 19716, USA d Geoscience and Environmental Engineering, Southwest Jiaotong University, Chengdu, Sichuan 610031, China e Institute for Environmental Management (IEM), Inc, Palo Alto, CA 94306, USA b
a r t i c l e
i n f o
Article history: Received 8 April 2015 Revised 20 June 2015 Accepted 22 June 2015 Available online xxxx Keywords: Methane emissions Field measurements Well radius of influence
a b s t r a c t A new in situ method for directly measuring the gas collection efficiency in the region around a gas extraction well was developed. Thirteen tests were conducted by injecting a small volume of gas tracer sequentially at different locations in the landfill cell, and the gas tracer mass collected from each test was used to assess the collection efficiency at each injection point. For 11 tests the gas collection was excellent, always exceeding 70% with seven tests showing a collection efficiency exceeding 90%. For one test the gas collection efficiency was 8 ± 6%. Here, the poor efficiency was associated with a water-laden refuse or remnant daily cover soil located between the point of tracer injection and the extraction well. The utility of in situ gas tracer tests for quantifying landfill gas capture at particular locations within a landfill cell was demonstrated. While there are certainly limitations to this technology, this method may be a valuable tool to help answer questions related to landfill gas collection efficiency and gas flow within landfills. Quantitative data from tracer tests may help assess the utility and cost-effectiveness of alternative cover systems, well designs and landfill gas collection management practices. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Methane (CH4) is an important contributor to global warming with a radiative forcing estimated to be close to 20% that of carbon dioxide (CO2) over the last two decades (IPCC, 2013). If the change in carbon storage due to climate change is taken into account the global warming potential of CH4 has been determined to be 34 times more than CO2 (IPCC, 2013). In 2012, landfill CH4 emissions represented the third largest source of CH4 emissions in the United States, behind natural gas systems and enteric fermentation (US EPA, 2014a,b). Budgets are needed to inventory landfill CH4 emissions. These can be expressed as production = recovery in gas wells + oxidation in cover soils + surface emissions + subsurface migration + D storage (Bogner and Spokas, 1993; Spokas et al., 2006), although the migration and storage terms are sometimes neglected (Börjesson et al., 2009). In this case, landfill CH4 collection efficiency is computed as recovery/production, where recovery is measured directly from the sum of all gas collection wells, and production is the sum
⇑ Corresponding author at: Air Quality Research Center, University of California, Davis, CA 95616, USA. E-mail address: [email protected] (R. Yazdani).
of recovery + oxidation + emissions (Börjesson et al., 2009) and may include + migration + D storage (Spokas et al., 2006). While this procedure has been used to evaluate the CH4 collection efficiency of an entire landfill, it is an expensive indirect measurement that because of cost is usually performed infrequently. In addition, this approach cannot be easily used to assess the efficiency of particular gas collection well designs, which may be installed at only a few locations. If instead the efficiency of landfill gas collection is assumed, CH4 production may be estimated from recovery data alone. This approach was recently applied to develop and improve U.S. EPA’s Landfill Gas Emissions Model (LandGEM) (Wang et al., 2013, 2015). In this case, the efficiency of landfill gas collection was based on expert judgment, using information about waste age, well density, and cover type on the landfill surface (Wang et al., 2013). Unfortunately, there can be significant uncertainty in estimates of landfill gas collection efficiency (Wang et al., 2015). Direct measurement of landfill gas collection efficiency may improve such modeling efforts. In addition, such measurements may help to enhance landfill gas collection by improving operation of landfill gas collection systems and design of gas collection wells. In two previous studies (Han et al., 2007; Yazdani et al., 2010) partitioning gas tracer tests were successfully performed to
http://dx.doi.org/10.1016/j.wasman.2015.06.032 0956-053X/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Yazdani, R., et al. Quantifying capture efficiency of gas collection wells with gas tracers. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.06.032
2
R. Yazdani et al. / Waste Management xxx (2015) xxx–xxx
measure waste moisture content in a full-scale bioreactor landfill. While the collection efficiencies of the injected gas tracers were computed, tests were too few to assess the variation of collection efficiency around the extraction well, the repeatability of measurements, and the relationship between collection efficiency and gas suction at the point of tracer injection. In this work in situ gas tracers were conducted that address these limitations. In these tests, a conservative tracer gas is injected at multiple locations near a gas collection well where gas pressure is measured. Using the known mass of tracer gas injected and the mass recovered from the gas collection well, the collection efficiency is determined. The collection efficiency is measured for specific locations surrounding a gas collection well, providing a picture of how collection efficiency varies in space and perhaps time for repeated tracer tests, due to changes in atmospheric conditions or landfill operations. As distinct from other approaches, employing gas tracers to determine landfill gas collection efficiency is a direct measurement. However, because this approach requires tracer injection at individual points in a landfill, the method determines the collection efficiency for these points only; multiple points are needed to assess collection efficiency surrounding a well. The traditional definition of CH4 collection efficiency is CH4 gas recovery/production. At any point in time it is likely production varies spatially within waste in an unknown pattern associated with waste composition and moisture condition. The average collection efficiency of tracers injected at multiple points within a waste mass may not be identical to CH4 gas recovery/production in the sampled region. Landfill gas may be generated in nonuniform patterns surrounding gas collection wells that cannot be precisely matched with tracer data: it is not possible to weight the collection efficiency of tracer gases with the corresponding production of CH4 at each tracer injection point, since CH4 production at these points is unknown. Thus, the gas tracer technology proposed here provides gas collection efficiencies that when integrated in space surrounding a well may not be identical to overall CH4 collection efficiency in the sampled region. Nevertheless, gas collection efficiencies from tracer tests at multiple points in a landfill cell provide a picture of spatially varying collection efficiency, where it is good and where it is poor, which should correlate with regions of good and poor CH4 collection efficiency. The maximum radius of influence (ROI) of a vertical gas collection well can be defined according to a methodology established by the US EPA (US EPA, 1996). Briefly, the ROI is the furthermost distance from a gas collection well where in situ gas pressures exhibit measurable suction from well vacuum. The ROI is the average distance determined from gas probes located along three radial arms 120° apart and at depth equal to the top of the screened well. Because precision of gas pressure measurement varies with instrument, the ROI is not well-defined and is generally smaller for less precise pressure measurements (Walter, 2003). Despite this limitation, we are unaware of data quantifying the collection efficiency for points within the ROI. Implicit in empirical well test methods for estimating landfill gas production (Walter, 2003) is that collection efficiency approaches 100% within the ROI. In this study the gas tracer technology outlined above was developed and tested for measuring the landfill gas capture efficiency at points surrounding a vertical gas collection well. This technology could be used to quantify the benefits of alternative landfill gas well designs, to track the efficiency of an existing collection system as a landfill cell moves from active to intermediate to final cover, or to evaluate various operational changes, e.g., modifications to the gas collection system to mitigate the effects of barometric pressure changes. Gas collection efficiency was assessed at points within the ROI of one well to illustrate how collection efficiency might vary within this region.
2. Materials and methods The focus of this work was the testing and development of an in situ gas tracer technique and included the following tasks: (1) design, construction, and installation of field testing equipment; (2) completion of multiple in situ tracer tests under varying pumping conditions and climatic settings; (3) measurement of gas pressure fields for gas tracer tests; (4) measurement of moisture conditions in cover soil; and (5) analysis of in situ gas tracer tests to determine gas well collection efficiency. The procedures and methodologies for Tasks 1–4 are described below. The procedures for analyzing gas tracer tests for Task 5 are discussed in detail in other publications (Han et al. 2006, 2007). 2.1. Field construction and instrument installation Field tests of the in situ gas tracer technology were performed at the Yolo County Central Landfill in Woodland, California. The test area was within a 6-hectare landfill cell (average waste depth of 9 m) that received 400,000 tonnes of municipal solid waste between 2003 and 2006. The average gas flow rate from this landfill cell from 10 gas wells during the field tests was 180 SCMH. During the field tests none of the gas wells within the 6-hectare area, other than test well D23 described below, were adjusted. The landfill cell was covered with an intermediate soil cover consisting of 50–60 cm of compacted clay soil (Capay silty clay) taken from a borrow site adjacent to the landfill. In some regions, an additional 15–18 cm of compost overlaid the cover soil. In situ gas tracer tests were conducted in the vicinity of vertical gas collection well D23, which was 9.1 m in depth and consists of a 10.2 cm diameter Schedule 80 PVC pipe perforated at the bottom 4.6 m. A schematic of the well design is shown in Fig. 1. The closest vertical gas well was 31 m away to the north. To measure the volumetric flow rate of landfill gas collected from gas well D23 during the tracer tests, a Roots Meter Series B3 5M175 flow meter (Dresser Inc., Houston, TX) was used with a measurement error
Fig. 1. Schematic of vertical gas collection well D23 used in the field. Not to scale.
Please cite this article in press as: Yazdani, R., et al. Quantifying capture efficiency of gas collection wells with gas tracers. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.06.032
R. Yazdani et al. / Waste Management xxx (2015) xxx–xxx
