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Facts about landfill gas

Landfill gas is like a big biogas unit. When the waste is put at the dump the proces of breaking it down starts. When the organic material decompose it transforms into gas. This proces continues for years.

The gas contains typical of 50 % methan, 45 % carbooxide and 5 % nitrogen. The about of energy from 1 ton waste in gas form is about 500 kW/t. How fast a waste dump decompose can variate from site to site due to lokal factors.

The half-life is for a typical danish dump about 15 years.


Brief description of our concept for gas recovery systems and power production technologies.


The company Deponigas  ApS was established in 1999 in recognition of the fact that the existing landfill gas plants in Denmark used technologies that were too automated and made ​​use of expensive materials and technologies in a manner that was completely unnecessary for the purpose. Consequently, the plants were too expensive in construction and operation.
Our concept is therefore purpose designed with a  low-tech approach and as a result, cheap and robust.

Gas recovery system:

Boreholes :
In the beginning we used ´ boreholes (wells)´ that were made with a battering ram technique using a drill bit with a diameter of 150mm. Later we moved to using auger drilling with diameter of 150 mm at a depth of typically 8-10 meters. Auger Drilling has the advantage that we can see the waste we drill in and the waste around the borehole is not compacted making it easy to withdraw the gas.
Our philosophy is: many wells = large surface coverage .
Suction pipes consist of diameter 40 mm perforated PEL tubing with air-tight seal on the upper 1 meter of borehole, to avoid air being sucked in.

Gas transmission is also done using PEL tubing, typically by 25 to 40 mm in diameter piping depending on the amount of gas and transmission distance.
Boreholes are preferably used at landfills that are completed with a final layer of clay and topsoil.

Gas collection from horizontal trenches:
They are used where there  still is landfilled being deposited. In this case a trench is dug and a drain hose of diameter 80-110mm.is placed, then it is covered up again with the dug up waste. The gas transmission pipe has a diameter 32-40mm. PEL hose. The transmission pipe is inserted in to the drain hose a few meters into the drain hose and then on its way to the manifold it is covered for protection against traffic.
The gas produced from each well or trench is piped to a manifold consisting of manual ball valves. Here it is possible to adjust the suction from each pipe separately. There are also outlets for analyzing gas with hand held gas analyzer.
From the manifold the gas is sucked by a speed controlled suction pump and then it is passed through a gas meter. By regulating the gas flow at the manifold the amount of gas collection can be controlled. Thus it is possible to estimate the energy potential of the landfill site and there by the size of the motor power generator required.

In Denmark it is often only the electricity production that is relevant. The heat is wasted as it requires a local consumption near the site.



Test pumping
Test pumping period is normally 2-3 months. This is sufficient to find the balance between gas potential of the landfill and the amount of gas we can harvest. In the case of a landfill site where waste is still being deposited it is of less importance if deposited waste has a gas potential.

Motor generator technology:
Our other distinctive characteristic is that we use the dual-fuel principle (diesel + landfillgas ) on our motor - generator sets . A small amount of diesel fuel is used to ignite the gas and to cool the injector nozzles. This avoids problems with unstable engine operation due to large fluctuations in gas quality which is caused due to varying atmospheric pressure. This also allows us to use gas with lower methane content. Spark ignition engines require high methane content in order to run smoothly.
We regulate the diesel used to a minimum so that the entire electricity production is propelled using the landfill gas. As mentioned the diesel amount used is just enough to ignite the gas but not enough to produce power. By using traditional diesel engines converted to duel fuel operation the investments are also affordable. Diesel engines are normally cheaper to purchase than purpose built gas engines with spark ignition.
The electricity production is normally sold through the public electricity grid and we receive payment for it.
The motors generator sets are typically placed in a 20 ´ steel container together with the control systems and the necessary alarms. Placed in the container is also the gas pump , safety valves, pressure monitors and gas flowmeter etc.

The newest plants all have remote control.


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