Methane collection efficiency of horizontal landfill gas collectors.

Abstract

Pumping test and closed flux chamber technique were used to evaluate the performance of horizontal landfill gas collection system at the Rachathewa sanitary landfill, Samutprakan, Thailand. Two investigated areas consisting of three horizontal extraction wells each with a total length of 150 meters with a well spacing of 15 m were used. The closed flux chambers were used to measure the surface landfill methane emissions before and during pumping tests. Pumping tests were done for a period of two weeks. The gas adjustment was tuned to maintain landfill gas (LFG) quality of greater than 50 % C[H.sub.4] and less than 2 % [O.sub.2]. Portable landfill gas analyzer and pre-calibrated orifice plates were used to control composition and collection rates of landfill gas from the collection system, respectively.

The average spatial methane emission rate without extraction process was 82 g/[m.sup.2]/d and with extraction process was 8.5 g/[m.sup.2]/d. The results showed that the spatial distribution of methane emissions without extraction process is very high and is reduced when the extraction process is applied. The efficiency of horizontal landfill gas extraction system was markedly above 80%. The collection efficiency from this study can be used for evaluation of landfill gas to energy projects. Furthermore, from the methane emissions data, it is recommended that in order to distribute suction force into the middle area to improve the collection efficiency, the position of wellhead should be offset about 10 to 15 meters from the end of collector. Thus, the collection of LFG by forced extraction would be an effective option in order to utilize LFG as well as reduce the global warming and local air pollution problems.

Keywords: landfill gas; methane emissions; pumping test; flux chamber, horizontal collector.

Introduction

Landfill gas (LFG) is formed as a natural byproduct of the anaerobic decomposition of wastes in landfills. Typically, LFG is composed of about 50% methane, 45 % carbon dioxide, and 5% of other gases including hydrogen sulfides and volatile organic compounds. LFG is thought to be released from six months to two years after waste is placed in the landfill [1]. Methane is a potent greenhouse gas (GHG), with 21 times the global warming potential of carbon dioxide. Estimates indicate that about 13% of methane emissions released to the atmosphere in 2000 were from landfills [2].

LFG can contribute to malodor and present health and safety hazards if it is not well-controlled. To recover its energy value and minimize its pollutant effects, many landfill sites have installed LFG recovery and utilization systems. Recovery of 100% of the generated gas is generally considered infeasible due to the permeability of wastes and recovery system inefficiencies as well as installation timing. There is lack of information about the recovery efficiency of LFG. In Thailand, the absence of data on recovery efficiency makes it difficult for landfill site owners and developers to set realistic target and establish reference for the use of LFG for energy projects.

The purpose of this study is to evaluate the performance of horizontal landfill gas collection system at the Rachathewa sanitary landfill, Samutprakan, Thailand by using pumping tests and closed flux chamber technique. Two major aspects have been studied--C[H.sub.4] emission rates in the presence and absence of gas extraction.

Methodology

Site Description

The Rachathewa sanitary landfill site is located 30 km east of the Bangkok area and receives about 3,500 tons/day of municipal solid waste. The site operated from 2000 to 2003 and occupies 40 hectares including a landfill and ancillary facilities necessary to support its operation.

The experimental cell is located at the one part of disposal area. The disposal area has been filled in three zones and the waste has primarily been placed using loaders, bulldozers, and a landfill compactor. At the present time (2006), there are approximately 1,902,380 metric tons of solid wastes in place.

LFG Collection System

The two common ways to recover LFG are vertical extraction wells and horizontal collectors. The standard and most commonly used is the vertical extraction well. The well is drilled into the landfill at spacing typically ranging from 45 to 90 m. Two to 8-inch diameter pipes (typically PVC or HDPE) are placed in the holes, which are backfilled with 1-inch-diameter, or larger, stones. The pipe is perforated in the lower section where the LFG is collected.

Horizontal extraction collectors or trenches may be installed instead of/or in combination with vertical wells to collect LFG. They consist of excavated trenches (similar to a pipeline trench) which are backfilled with permeable gravel. Perforated, slotted, or alternating diameters of pipe are installed in the trench. Horizontal extraction collectors are less expensive than vertical extraction wells and are particularly suitable for installation in active filling areas. The advantages of a horizontal extraction collector are low effects from high leachate level problem in landfill, less obstruction for landfill operations caused by collector headers and easy installation. The disadvantages of a horizontal extraction collector are high effects from waste settlement and a low recovery efficiency rate per well [3].

The LFG collection system should be used in concert with good leachate management practices. Leachate accumulation within the refuse can dramatically impact the rate of LFG recovery because liquid in the extraction well and collection trenches effectively restricts their ability to collect and convey LFG [3].

Field experiences indicate that horizontal extraction collectors are more suitable for Thai landfills compared to vertical extraction wells [4]. The main purpose of using the horizontal extraction well is the very high leachate level in Thai landfills. Figure 1 provides a schematic of the typical horizontal collectors at the site.

[FIGURE 1 OMITTED]

Size of Experimental Cell

The two investigated areas had three horizontal collectors with a total length of 150 meters. Spacing between the wells was 15 meters. Each cell covers a footprint area of approximately 45m x 50m. The cell covers three horizontal gas extraction wells. The collector extended from the outside edge into the landfill with a 3% slope. At the outside edge, leachate drainage was provided using a PVC pipe. The leachate drain was 12 meters long and had a diameter of 100 mm. The location of two investigated areas is shown in Figure 2.

[FIGURE 2 OMITTED]

Methane Emissions Rate

Gas samples were collected using a closed acrylic chamber (length 40 cm, width 40 cm and height 10 cm) that was equipped with an electronic thermometer and a sampling port. The bottom of the chamber was placed on the soil surface with 5 cm inserted into the soil. The height between soil surface and bottom level of sealed trench of basement was measured. To protect from air disturbance, the water was filled to the trench of basement. The flux chamber was placed to the basement and gas sampling started. The concentration of methane within the box was measured at short time intervals (1 minute) by syringe and conveyed to the vacutainer over a period of 4 minutes.

Methane concentration was measured by a Shimadzu 14A gas chromatograph (Shimadzu Co., Japan). The rates of methane emissions were calculated by fitting linear regression to the difference in the methane concentrations and adjusting for the chamber volume and area covered following the equation 1.

Q = V/A (dc/dt) (1)

where:

Q = flux density of the gas (mg.[m.sup.-2][s.sup.-1])

V = chamber volume ([m.sup.3])

A = chamber footprint ([m.sup.2])

dc/dt = rate of change of gas concentration in the chamber with time (mg.[m.sup.-3][s.sup.-1])

For measuring methane emission rates, 100 sets of closed flux chambers were installed on the final cover soils at each measurement site (both with and without extraction process). The chambers were placed in a grid pattern in the 1st and 2nd areas having dimensions (x - y) of 7.5 m x 16.7 m and 7.5 m x 10 m, respectively.

LFG Extraction

LFG extraction by root blower at the Rachathewa power plant had been performed for a period of two weeks. The manifold at each wellhead was tuned to maintain landfill gas (LFG) quality greater than 50 % C[H.sub.4] and less than 2 % [O.sub.2]. The composition and production rate of landfill gas from the well system was measured everyday using a portable LFG analyzer (GEM 2000) and pre-calibrated orifice plate.

Spatial Interpolation 

Mapping software Surfer by Golden Software was used to analyze the geospatial distribution. Kriging model was applied to map the results, and the volumetric value of the contour map was produced by volume and area integration algorithms in Surfer.

Results and Discussion

The results showed that the methane emissions without extraction process were in the range of 1.93 to 351.44 g/[m.sup.2]/d in the 1st area and 0.51 to 754.74 g/[m.sup.2]/d in the 2nd area. The spatial average methane emissions without extraction process were 66.48 and 97.67 g/[m.sup.2]/d for the 1st and 2nd area, respectively. The methane emissions with extraction process ranged from 0.00 to 24.65 g/[m.sup.2]/d in the 1st area and 0.00 to 268.09 g/[m.sup.2]/d in the 2nd area. The spatial average methane emissions with extraction process were 4.18 and 12.91 g/[m.sup.2]/d for the 1st and 2nd area, respectively. The efficiencies of the collector ranged from 9.93 to 100 % and 5.96 to 100 % for the 1st and 2nd area, respectively. The overall spatial efficiencies of horizontal landfill gas extraction system were about 82% in both the areas.

Methane emissions in the 1st and 2nd area with and without extraction process are shown in Figure 3. The efficiencies of horizontal landfill gas extraction system in these two areas are presented in Figure 4.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

These tests showed that the spatial distribution of LFG emissions was lower in the 1st area than in the 2nd area. This trend was also observed when LFG was being extracted. However, in the latter case, the spatial distributions of LFG emissions were lower than without extraction for both areas.

Utilization of geospatial methodologies to determine the whole landfill emission rates in both areas showed that the amounts of LFG emissions from the landfill surface were about 20% higher than the amount of LFG generation with extraction process. When LFG was extracted by blower, the LFG emissions rates rapidly decreased. In order to improve the collection efficiency it is recommended that the position of wellhead should be offset about 10 to 15 meters from the end of collector. This will improve the suction force distribution in the middle of extraction area.

Conclusions

Using pumping tests and closed flux chambers technique to evaluate the performance of landfill gas extraction system and methane emissions revealed significant methane emissions and the effect of LFG extraction on methane emissions. The results showed that the spatial distribution of methane emissions without extraction process is higher than when the extraction process is applied. The spatial average methane emissions without extraction process were 66.48 and 97.67 g/[m.sup.2]/d in the 1st and 2nd area respectively. With extraction process, they were 4.18 and 12.91 g/[m.sup.2]/d in the 1st and 2nd area respectively. The average spatial efficiencies of horizontal landfill gas extraction system, determined from the collection efficiency at each sampling point, were about 82%.

Furthermore, from this obtained information, the surface LFG emissions patterns can be progressively developed in the extraction well system design process. In order to improve the collection efficiency it is recommended that the position of wellhead should be offset about 10 to 15 meters from the end of collector to improve the suction force distribution into the middle of extraction area.

The conservative value for LFG recovery by using horizontal LFG collection system for evaluation in landfill gas to energy project planning can be 80%. This will significantly reduce the environmental impact on landfill emissions. Thus, the utilization of LFG by forced extraction would be a very good solution to maximize LFG recovery and so to use CH4 as biofuel to mitigate GHG emissions as well as reduce odors in neighborhoods surrounding the landfill [6].

References

[1] U.S. Environmental Protection Agency (EPA), 1997, "Opportunities for Landfill Gas Energy Recovery in Colorado," EPA 430-B-97-036, Washington, DC: EPA.

[2] U.S. Environmental Protection Agency (EPA), 2005, "Global Emissions Report, Washington," DC: EPA.

[3] The World Bank, 2004, "Handbook for the Preparation of LFG to Energy Projects in Latin America and the Caribbean," Available online at: www.bancomundial.org.ar/lfg

[4] Eam-o-pas, K. and Panpradit, B., 2003, "Landfill Gas Recovery Using Horizontal Collectors in Thailand," Fourth International Conference of the ORBIT Association, Perth, Australia.

[5] Wang-Yao, K., Towprayoon, S. and Wangyao, P., 2005, "Landfill Gas to Energy in Thailand: Case Study at Rachathewa Power Plant," Third Eco-Energy and Materials Science and Engineering Symposium, Chiangmai, Thailand.

[6] Environment Agency, 2004, "Guidance on the Management of Landfill Gas," Available online at: www.environment-agency.gov.uk.

Komsilp Wang-Yao (1) *, Sirintornthep Towprayoon (1), Chart Chiemchaisri (2), Shabbir H. Gheewala (1) and Annop Nopharatana (3)

(1) The Joint Graduate School of Energy and Environment, King Mongkut's University of Technology Thonburi, Bangkok, Thailand 10140 E-mail: [e-mail], [e-mail] and [e-mail]

(2) Department of Environmental Engineering, Faculty of Engineering, Kasetsart University, Bangkok, Thailand 10900 E-mail: [e-mail]

(3) Pilot Plant Development and Training Institute, King Mongkut's University of Technology Thonburi, Bangkok, Thailand 10150 E-mail: [e-mail]