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Elements of a Landfill Gas System
There are several key elements to a landfill gas recovery and utilisation system:
- The wellfield to collect the gas from the landfill
- Gas pre-treatment plant to clean the gas for combustion
- The power station to generate the renewable electricity
- Connection to the electricity grid and
- A Flare to destroy any gas not combusted in the engines
This is shown diagrammatically in Figure 1 below
The engineered landfill gas extraction system comprises the installation of a series of vertical wells on a grid layout to collect the gas, being post closure and during construction of an active cell.
The design of the grid is determined by the landfill characteristics, these being:
- Waste composition and moisture content, including amounts of organics and inert material deposited into the landfill;
- The land filling operations including:
- Type of compaction equipment used and number of passes;
- Cell construction: Time taken to fill the active cell? Suggest maximum of 18 months based on delivery rates;
- Age of the waste: Best gas comes from the new waste, so prefer to install wells as landfilling operations progress;
- Surface area and side slopes: How accessible is the surface to install wells post closure?
- Depth of waste: Greater than 20 metres is desirable, whilst greater than 10 metres is an absolute minimum and;
- Type of final cover provided by the landfill operator: Clay goes through a seasonal wetting and drying cycle and gas escapes when it is dry. Plastic liners are better as there is less opportunity for the gas to escape.
Installation of horizontal wells is preferable to capture gas that is generated as the cell is being constructed. Gas generated by waste placed above the horizontal system is drawn down to the horizontal collectors and away from the surface.
These wells are connected by pipes to the collecting manifold, which delivers the gas to the power station via a vacuum pump. The wellfield operates under a slightly negative pressure to draw the gas out.
Gas delivered to the power station has to be prepared for combustion in engines designed for landfill gas. The gas pre-treatment process will vary depending on the composition of the landfill gas collected.
Once the gas has been cleaned ready for combustion, it is delivered to the engines. The number of engines and generating units will depend on the volume of gas being delivered from the landfill wellfield. New engines typically have the capacity to generate from 1MW to 2MW of electricity. In LGP’s case, we have a mix of Caterpillar engines generating from 650kW to 1MW of electricity at our three power stations thereby providing added flexibility and the opportunity to maximise electricity generation from the available gas supply.
It is important to continually monitor the wellfield to ensure that optimum gas being produced is captured and is available for combustion, thereby maximising electricity generation.
Electricity generated is passed into the electricity grid for use by customers.
Generally this type of landfill gas management system will have a flare, which is available for use in the event of the power station being out of action. This ensures compliance with the environmental requirements to dispose of the landfill gas generated in an acceptable environmentally friendly manner.
Power station operations can be monitored from a remote central location and thus, the stations can be down the street, in another state or even another country. On site personnel are required to attend to wellfield monitoring and undertake routine maintenance works.
Leachate can become a problem with gas extraction if it is not managed appropriately. Any leachate which is collected in the sealed wellfield gas extraction system and associated pipework is drained to designated low points and discharged back into the landfill or into the leachate collection system.
In 2008, a total of US$155 billion was invested in clean energy. This represents 10% of all infrastructure spending. Landfill gas recovery and its utilisation would represent a very small portion of this expenditure, yet its benefits to the environment are substantial.