Of the three typical ways of removing methane from landfills — venting, burning, and extraction — only the latter properly disposes of the potentially hazardous gas and offers the option of selling it as fuel. Over the last two decades, 594 U.S. gas-to-energy sites have taken advantage of this benefit, generating 1,813 megawatts of electricity and 312 million standard cubic feet a day of gas, and lowering greenhouse gas emissions 30%.
Methane extraction, however, is a complicated and sensitive process that involves drilling wells into landfill cells to extract and move the methane, a naturally occurring byproduct of decomposing organic waste. Methane poses two hazards. Although it’s in the air we breathe, it’s a health hazard at high concentrations because it displaces oxygen. And it’s dangerous because it’s highly flammable and potentially explosive.
As a result, engineering and installing an extraction system requires expertise that virtually all owners contract out. Here’s what those specialty contractors do and why.
Installing methane extraction wells
Methane gas extraction wells are constructed by using a drill rig to install boreholes, typically at least 24 inches in diameter, into the landfill to remove the trash and allow for installation of a well screen and casing. Crews use bucket-type drill rigs with the bucket attachment on the end of the drilling stem or a kelly bar attached and drilled into the trash, which is extracted. They remove trash via the borehole one bucket at a time.
Because landfills are filled from the bottom up, the deeper the borehole, the older the trash, and the greater the concentration of methane. Crews monitor the open borehole for low oxygen levels, hydrogen sulfide, and explosive gases.
Boreholes more than 100 feet deep are common. Crews make sure they know the depth so they don’t puncture the lower liner or other bottom seal. They install the well, which consists of a slotted or perforated well screen with a cap on the bottom and solid casing to the surface, at the center of the borehole. Depth of the well and length of the well screen depend on where the methane has accumulated.
An expansion joint installed within the well material allows for movement when the landfill settles. Because the well screen and casing — as well as all other fittings, hoses, or piping — must withstand landfill contaminants, they’re made of high-density polyethylene (HDPE) and/or polyvinyl chloride (PVC) plastic pipe.
Next, crews fill the annular space between the well screen and casing and the borehole wall with gravel or rock sized to allow the gas to move easily from the landfill into the well. They place the aggregate from the bottom to near the top of the borehole to allow for maximum extraction. To keep methane from escaping through the top of the borehole, they install a borehole seal of very low permeability or impervious materials, such as bentonite or an HDPE liner seal.
Now that the gas has a path through which to move, crews install a well head at the top of the well casing. If the concentration of methane is minimal and deemed a safe level — and state regulations allow — they install a passive well-head, which vents the gas to the atmosphere. If regulations don’t allow passive venting, burning off the methane through individual wells with a flare stack may.
Collecting and conveying methane
If the landfill is of substantial size and the anticipated quantity of methane precludes flare stacks, or if the gas will be used for other purposes, a series of extraction wells is installed and manifolded into conveyance piping for disposal at a larger flare station.
Well heads typically have a valve, flow measuring device, and sample ports to allow for sampling and temperature testing. This is necessary to regulate the flow from individual wells so as no to over-extract from any one well. Each well head is plumbed to the main collection pipeline with a flexible hose. As with all other materials, these well-heads and hoses must be compatible with methane and are specialty items.
Once on the surface, the piping is installed continuously downhill. Because of its construction, HDPE pipe is usually used for the conveyance pipe and is fused together for a permanent installation. HDPE compensates for temperature-driven expansion and contraction.
Leachate collection and pumping
If the piping can’t be installed downhill, crews install a leachate sump and pump on any low spots. If these points fill with leachate — the liquid condensate from landfill gases or any other liquid that leaches into the landfill — the methane won’t be able to move, creating a safety hazard.
The sump is generally made of the same material as the piping, with HDPE the most common. The sump generally is installed into a borehole belowgrade and is attached directly into the pipeline.
Crews install a corrosion-resistant pump into the sump. To prevent a potential fire or explosive hazard, it’s usually a pneumatic (air) rather than electric pump. The liquid falls into the lower section of the sump and is pumped away, allowing the gas to continue on its path to disposal.
On large landfills, the conveyance pipe is generally connected to an engineered blower and flare station to create a vacuum that pulls the methane to the station. The gas is then injected into the flare. The flare portion burns the methane and, in some systems, can dispose of the leachate.
On landfills that are large enough, extraction systems also can allow the gas to be converted to fuel. Local companies buy this fuel to power equipment or run generators that, in turn, provide electricity to the surrounding community.
—Jerry Soto (email@example.com) is project manager for Houston-based Griffin Dewatering Corp. Visit www.griffindewatering.com.