Landfill Gas Systems

Transcription

1 Landfill Gas Systems A General Overview ASTSWMO STATE SOLID WASTE MANAGERS CONFERENCE Scottsdale, Arizona September 12-14, 14, 2005

2 Landfill Gas Management Issues/Objectives Topics to be Covered Reasons why LFG must be collected Compliance with regulations Environmental safety Environmental concerns

3 Why Install a Landfill Gas Collection System? Subsurface migration control Emissions control - EPA regulations Odor Control - reduce hydrogen sulfide emissions Control gas pressures to protect cover system Energy recovery Prevent LFG from contaminating groundwater Multiple goals It s important to consider your objectives when performing LFG design

4 LFG Basic Concepts LFG produced by decay of MSW LFG consists of: About half and half methane and carbon dioxide Small amounts of VOCs, nitrogen, hydrogen sulfide Explosive at 5% to 15% methane content LFG production depends on: Amount of waste in place Age of MSW Moisture content Production leads to pressure buildup within the landfill

5 Elements of an LFG Collection and Control System LFG collection points: Vertical extraction wells Horizontal collectors/trenches Connection to existing vents, wells, etc. Network of interconnecting piping Elements of condensate management Flow control LFG blower/combustion device (flare, engine, etc.)

6 Vertical Extraction Wells Versus Horizontal Collectors Vertical wells allow greater operating flexibility Vertical wells - better LFG quality Horizontal collectors usually installed during filling Horizontal collectors typically more sensitive: Differential settlement Leachate flooding Both can be used while filling occurs Horizontal collectors where waste is shallow

7 Vertical Extraction Wells Design Parameters/Guidelines In-refuse wells - 75% of the refuse depth Depth of in-soil wells varies Groundwater level Bottom of refuse Depth of gas migration Boreholes typically 24 to 36 diameter Casing is generally PVC or HDPE Bottom perforated - start 20 feet below ground surface Spacing depends upon radius of influence 200 to 400 foot spacing on landfill Tighter spacing in soil

8 TOP DECK EXTRACTION WELL LANDFILL COVER SIDE SLOPE EXTRACTION WELL LANDFILL MSW PERIMETER EXTRACTION WELL (IN LANDFILL MSW) PERIMETER EXTRACTION WELL (OUTSIDE LANDFILL MSW) SHALLOW DEEP PERIMETER MONITORING PROBE

9 Typical Vertical Extraction Well Bentonite seal prevents air infiltration Wellhead incorporates: Flow control valve Pressure monitoring port Flow monitoring device (optional) Thermometer (optional)

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11 Horizontal Collectors Design Parameters/Guidelines Installed as refuse is being filled Typical spacing 100 to 200 feet horizontally 40 to 60 feet vertically Perforated pipe in a gravel trench Pipe is typically HDPE, or PVC with alternating diameters Can be installed in single layer just below cap - surface collectors

12 Typical Horizontal Collector Arrangement

13 Typical Horizontal Collector Arrangements

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15 Prevention of Subsurface LFG Migration Explosive hazard (safety and liability) Contact with hazardous compounds (safety and liability) Federal driver is Subtitle D 5% limit at property boundary Quarterly monitoring State and local regulations are often more definitive than Subtitle D Probe design is based on site specific circumstances Typically located at property boundary Typical horizontal spacing is generally 200 ft to 1000 ft (depending on adjacent land use) Probe depth is normally to the depth of refuse or to groundwater

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17 Uncontrolled LFG Concentration Impact

18 LFG Control Pressure Impact

19 LFG Control Concentration Impact

20 LFG Surface Emissions LFG is a source of VOCs and hazardous air pollutants (HAPs). VOCs and NOx combine to form ozone, an ambient air criteria pollutant HAPs are a subset of NMOCs LFG is a major source of greenhouse gas emissions Methane and carbon dioxide are greenhouse gases Methane has 21 times the potency of carbon dioxide on a mass basis LFG contains odorous compounds Regulatory drivers are NSPS/HAP regulations, Pre- Construction Permits

21 Passive Vent Flare

22 LFG Adverse Impact on Landfill Caps Soil Cover Damage to vegetation (aesthetic) Damage possibly results in increased erosion Membrane Cover LFG can produce a gas bubble under the membrane Bubble can cause membrane damage

23 NMOCs in LFG can Contaminate Groundwater Problem is most prevalent in dry climates, unlined landfills and landfills with no or inadequate LFG collection systems Three transfer mechanisms Direct LFG to groundwater contact Condensation of LFG in vadose zone (below and beyond landfill) Condensation of LFG in vadose zone plus washdown (beyond landfill)

24 Direct Transfer of NMOCs

25 Transfer of NMOCs by Condensation

26 Transfer of NMOCs by Condensation and Washdown

27 Landfill Gas Management Issues/Objectives Summary It is necessary to collect LFG to: Prevent subsurface migration Reduce surface emissions Protect groundwater Protect landfill cover