Main methods for detecting biogas composition
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There are austenite gas analysis methods, gas chromatography GC analysis methods, thermocatalytic element detection methods, and infrared detection methods. 【Austenite composition analyzer】: Sodium hydroxide solution is used to absorb CO2, while a pyrogallic acid alkaline solution is employed to absorb CO2 as well; CH4 is measured through a combustion method followed by an absorption process, thereby determining the concentrations of CH4, CO2, and O2. This method has a simple structure, but it requires the collection of representative gases and does not allow for on-site analysis. Additionally, a large amount of reagents must be purchased each year, the staff need to be trained, and the analytical testing involves considerable labor intensity.
【Gas Chromatography GC Analysis Method】: This method involves separating the sampled gas in a chromatography column based on differences in their physical adsorption properties, and then using a TCD sensor to analyze CH4, CO2, and O2. It is a simple testing method, but the chromatography equipment is relatively expensive; sampling is required, and analysis cannot be carried out on-site.
【Thermocatalytic detection method】: A Wheatstone bridge is formed by a thermocatalytic (black and white) element, a compensation element, and bridge arm electrodes, to which a constant voltage is applied. Since the framework of the thermocatalytic element is made of platinum wire, it heats up when current flows through it, resulting in a temperature of around 500 degrees. When gas is present, the gas comes into contact with the surface of the catalytic element, triggering a chlorination reaction that generates a large amount of heat; this raises the temperature of the catalytic element, increases its resistance, and causes an unbalanced voltage to be generated by the bridge, thereby indicating the concentration of methane being measured. The advantages of this method are its low cost, making it suitable for detecting whether methane levels in coal mines exceed safe limits; it is widely used in portable gas analyzers as well as fixed gas sensors. The disadvantages are: it requires frequent calibration, once every 7 days; it has a lifespan of only one year; its accuracy is low (10%); its lifespan is short, and it can only measure gas concentrations below 0–4%. High concentrations can poison the sensor or even render it unusable; in addition, CO2 cannot be measured.
【Infrared detection method】: Taking advantage of the fact that CH4 and CO2 gases absorb mid-infrared and far-infrared wavelengths at 3.4 um and 4.26 um respectively, the concentrations of CH4 and CO2 are determined through changes in infrared absorption rates; simultaneously, electrochemical sensors can be used to determine the concentrations of O2 and H2S. It is currently the preferred instrument for measuring biogas abroad. In China’s CDM projects, infrared biogas analyzers are required to determine the composition of biogas. Their advantages include the ability to conduct measurements in real time online, no need for gas bag sampling, no consumption of consumables, high measurement accuracy, and no need for frequent calibration. The measurement range is 0–100%; these analyzers can be used to measure both the gas composition in biogas digesters and biogas leaks.