Methane Flux Boxes for Monitoring Landfill Gas Cap Emissions
The UK Environment Agency may require that the landfill gas methane emissions from a landfill restoration cap are monitored regularly, so that areas where excess gas might be escaping, can be found, and the leak repaired.
In the UK FIDs are frequently used to measure gas concentrations close to the surface in
order to detect leaks. A typical FID survey involves the monitoring technician walking along predetermined lines, monitoring the gas concentration continuously. The method is mostly suitable for the measurement of specific point emissions and remains the most appropriate method for identifying defects in the cap.
FID with Flux Box Methods
Flux boxes can be used to measure the flow of methane from a surface, i.e. the rate of change in emission concentration over time. They are used to produce flux box surveys, the objective of which is to quantify the total release of methane from the surveyed area.
There are two different types of flux box available to do this, the static closed chamber and the dynamic closed chamber.
Whichever type is used, the box is placed onto the ground open side down with the open side temporarily sealed to the ground. The concentration of methane within the box is measured at short time intervals over a period of up to an hour. Once the data have been collected the methane flux is obtained by assessing the rate in increase in concentration over time.
This system is very simple and straightforward to perform and the accuracy is simply a function of the number of data points as well as being fairly cheap. Large numbers of monitoring points are needed.
Flux box surveys are currently used in the UK for determining emissions from capped areas of Site.
The flux box method best at measuring low landfill gas fluxes and thus is suited to measuring capped landfill site methane emissions. It is not practical for use on working operational areas of the landfill.
Taking measurement at a lot of different locations is very time consuming as well as labour intensive. It is therefore not an ideal method for use on the current size of landfill sites.
Flux box landfill gas emission monitoring must be done by technicians that understand the limitations of the technique, otherwise large errors are possible. Heating from solar warming or cooling by wind chill can lead to plus or minus 100% in measurement error.
Similarly, waterlogged ground or high wind speeds can change the measured flux by similar amounts. A typical survey takes two to three weeks work, or more by a technician and an assistant, plus data analysis costs.
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