Infrared methane sensors in landfill gas monitors

Supplier: Thermo Fisher Scientific
05 August, 2013

What is landfill gas? The waste within a landfill will consist of a wide variety of substances, but a large proportion will be biodegradable.

This will include animal and vegetable matter, paper and wood. These substances can be decomposed by micro organisms within the landfill and this breakdown process produces gas. Landfill gas can be a complex mixture of gases, but a few gases predominate. Initially, carbon dioxide is the main gas, but there can also be significant quantities of hydrogen.

Methane is produced during the major part of the decomposition process. Many other gases can be produced in trace amounts and the exact composition of the gas will vary between different landfill sites, different parts of the same site and over time. Infrared Absorption Infrared absorption is the primary method for measuring methane and carbon dioxide. Other newer technologies are emerging (laser dioxide) and are available nowMost gases absorb radiation in the infrared region.

The wavelength of radiation that is absorbed is determined by the natural vibration frequencies of the molecule. These natural frequencies will depend on the bond strengths, molecule size and shape and mass of the atoms involve. Thus different gas molecules have different natural frequencies and will absorb infrared radiation of different wavelengths. This property is used in infrared absorption to select a specific gas for analysis.

By using infrared radiation of the same wavelength as the absorption band, the technique can be made specific to a particular gas - a useful property when analysing one gas within a mixture. Infrared radiation from a source is passed through the gas to be analysed.

Several infrared detectors are positioned to measure the amount of infrared radiation that has passed through the gas. A filter that is tuned to the wavelength of interest is positioned in front of the infrared detector. This then makes that particular detector sensitive to the gas of interest.