3. Methane (CH4) monitoring

#### 3.1. Introduction

With the increasing attention to environment, energy, and safety, natural gas has gradually replaced coal as the main energy source in China, and its use has been increasing year by year. The extraction, transportation, and storage of natural gas have become an important part of social development. Equipment safety and high efficiency operation in gas transmission station are the keys to ensure the natural gas transportation. Once it is released, the serious safety accidents such as energy waste, environmental pollution, fire, and explosion will happen [42], which would directly threaten the safety of life and property of the countries and people [43].

The main component of natural gas is methane, accounting for 90%, and also contains a small amount of ethane, acetylene, butane, carbon dioxide, carbon monoxide, hydrogen sulfide, and so on. Traditional natural gas leakage detectors include flame ion detectors (FID), electronic detectors, electrochemical catalytic combustion detectors, and infrared absorption detectors [44]. However, these detectors are self-charging and have potential safety problems in the application of flammable, explosive, and other special environments. Moreover, these sensors are short in life, low in precision, poor in stability, and difficult in adjustment and often give the wrong results of measurements and misinformation. Recently, TDLAS technology has been widely used with the rapid development of narrow linewidth semiconductor laser technology [45]. The SRI International (Menlo Park, CA) company in America has developed a vehicular natural gas pipeline leakage detector, which improves the efficiency of pipeline leakage detection. However, they are all limited to the detection of methane and do not involve the detection of other gases in natural gas.

In view of the area of natural gas, gathering station is large, and the pipeline system of natural gas is gathered; point and portable measurement is not suitable in this situation. We designed an open, continuous detection and alarm system which has the characteristics of fast response speed and high detection precision based on TDLAS technology. Moreover, this system also detects ethylene, acetylene, and other gases, which improves the measurement precision and reduces the probability of false alarm.
