It is important that all landfill gas extraction wells (also known as gas extraction boreholes) are designed carefully in accordance with best practice. In this article we have tried to identify the most important aspect of design. We also recommend the United Kingdom reader to consult the UK Environment Agency Publication, “Guidance on the Management of Landfill Gas”.
All landfill gas collection and extraction systems must be designed individually for the site, taking into account the depth, shape and topography of the site. The designer will use the data from the gas production flow rate modelling and a pumping trial to size the flare, or utilization equipment..
Site Survey Requirements
It will be necessary to obtain an up to date topographical site survey of the landfill, and to be in possession of the fullest possible information on the size, location, and likely dates of infilling of any remaining landfill cells or phases to be developed.
Avoidance of Over Extraction – Causing Fires
The landfill gas systems installed all around the world rely on suction to that the system does not cause excessive amounts of air to enter the landfill gas. Great care must be taken to ensure that gas migration control. Particularly care near the site perimeter is needed to ensure that the extraction system does not draw in air. In severe cases this can cause fires in the waste by sucking excessive amounts of air into the waste mass.
Landfill Gas Extraction Well Design Concept
So, if gas migration is a potential problem on the site, the first requirement of the landfill gas collection system will be to encircle the site perimeter 40 m inside the perimeter. On this line wells will be spaced at 40 metre centres, and drilled to the base of the landfill, without penetrating the bottom liner. The intention is that these wells will intercept any landfill gas which would otherwise migrate out of the site. If there is no man-made or natural site development lining then a large amount of air will tend to enter this ring. At best this will make the gas extracted a low quality. So, for old gassing landfill sites without a clay lining, it is usually best to run separate pipework routes for the outer and inner landfill gas extraction systems.
In central areas of the landfill the wells may be set out at a wider spacing, and it is often preferred to space them at 50 metre centres. Each borehole is drilled to its full depth at approximately 325mm diameter and is then fitted with a high Density Polyethylene (known as HDPE or PE) pipe.
The actual pipe diameter chosen will depend upon the depth, as usually the pipe used will be smaller at the base so that the lower pipe lengths can be lowered through the larger diameter upper pipes. All well pipes are horizontally perforated at regular intervals, to allow leachate and landfill gas to pass through, except at the top where the pipe will pass through the landfill restoration capping material.
Leachate well pipework is not usually wrapped with a filter geotextile, which if used, tends to blind up with fine material and the flow into the well then reduces or ceases.
Twin wall construction, the inner wall smooth and the outer wall convoluted and with a maximum outer diameter of 160mm has been used by some, however, concern exists that this type of pipe is so flexible that there is a tendency for the wells made in this material to bend after installation more than for the solid HDPE pipe equivalent. The problem after gradual well pipe bending, is that the pump may become stuck at the bend.
Views on Gravel-Packing of Wells
Although there are differences in views and practice adopted by LFG well drilling contractors, it is normal that no gravel or any other material is placed in the annulus between the well rising pipe. Gravel packs tend to prevent differential settlement between the liner and the well head which is a thing to be avoided. Furthermore it has been observed that where a gravel pack has been used with certain types of liner the entire well pipe structure has sheared, as a consequence of being too rigid.
Landfill Gas Extraction Well Design – Well-Head Details
The top of the landfill restoration cap (or liner) is usually set at approximately 2.5m below ground level to accommodate this thickness of covering soils materials. The use of a well head with a sleeved top pipe is good. When settlement takes place the idea is that the wellhead slides over the liner and no stresses build up which otherwise usually brings problems at a later date. When finally installed with the lateral flange connections are placed on the floor of the pipe trench. They are sealed around with bentonite slurry.
Incentives are Available in Some US States for Landfill Gas Extraction and Collection
Various landfill gas project incentives exist for United States projects at the federal and state level. The Department of the Treasury, Department of Energy, Department of Agriculture, and Department of Commerce all provide federal incentives for landfill gas projects.
Typically, state incentives are in the form of tax credits, bonds, or grants. For example, the Renewable Electricity Production Tax Credit (PTC) gives a corporate tax credit of 1.1 cents per kWh for landfill projects above 150 kW. Various states and private foundations give incentives to landfill gas projects.
If gas extraction rates do not warrant direct use or electricity generation, the gas will be flared off.
Minimum Landfill Gas Collection Rate in the US and UK
One hundred m³/h is a practical threshold for flaring in the US. In the U.K, gas engines are used with a capacity of less than 100m3/h. Flares are useful in all landfill gas systems as they can help control excess gas extraction spikes and maintenance down periods. In the U.K and EU enclosed flares are mandatory at modern landfill sites. via en.wikipedia.org
In the UK and Europe landfill operators are required by environmental law to collect their landfill gas. They are not permitted to let it escape to the atmosphere and to at least flare it. This is required to keep the greenhouse effect (i.e. global warming effect) of landfill emissions as low as possible. No equivalent federal regulations exist in the US.
Landfill Systems (UK) provide a complete landfill gas system design, installation, and maintenance service. They are also experts in gas collection and flaring of low quality landfill gas, and below 100m³/h. Contact Landfill Systems for your landfill gas extraction system here.
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