Application
Modern landfills are monitored to ensure compliance with federal regulations under 40 CFR Parts 60, 61, and 63 (Federal Register/ Vol. 65, No. 201/Tuesday, October 17, 2000 /Rules and Regulations). PID Analyzers (hnu.com) has introduced its Model 108 Analyzer for measuring landfill gases.
The analyzer is used for soil gas monitoring, and measuring the near surface gases at a landfill site. Clearly, one on the biggest concerns at a landfill site is the methane (CH4) which has a lower explosive limit of about five percent. Areas outside the landfill can be checked to ensure that the CH4 is not migrating from the landfill to nearby homes. Since PID’s Model 108 is a four gas analyzer, the customer can choose either an electrochemical hydrogen sulfide (H2S) sensor or an atmospheric pressure sensor as an option. The analyzer can compensate for water in the sample up to 95 percent, but it is possible the sample can be saturated, so PID has added a water removal system to prevent liquid water from getting into the analyzer.
Technology
The Model 108’s infrared sensors have two interference filters and two temperature sensors: one at the absorbance maximum (3 microns for CH4), the other at the minimum. Each is in a leg of a Wheatstone Bridge circuit. If the sample is irradiated by an IR (infrared) source and CH4 is present, it will increase the temperature of the CH4 IR sensor and change the resistance. The change in resistance is proportional to the concentration. The carbon dioxide (CO2) works in a similar fashion but has an absorbance maximum at 4.3 microns. Figure 1 is an IR spectrum of CO2 where M is the measurement wavelength and R is the reference wavelength. Note that water absorbs strongly in the infrared and in Figure 1, the M and R wavelengths for CO2 are in a region where water does not absorb strongly.
If the sample is irradiated by an IR (infrared) source and CH4 is present, it will increase the temperature of the CH4 IR sensor and change the resistance. The change in resistance is proportional to the concentration.
The Model 108 has two dual beam (measuring and reference wavelengths) infrared analyzers for measuring 0–100 percent CO2 and CH4 and an electrochemical sensor for oxygen (0–30%). The electrolyte for the O2 sensor is acidic to keep the CO2 from building up in the electrolyte and destroying the sensor in a short period of time (months). The gas composition of the landfill gas is shown in Figure 2.
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