MQ-9 gas sensor has high sensitity to Carbon Monoxide, Methane and LPG. The sensor could be used to detect different gases contains CO and combustible gases, it is with low cost and suitable for different application.It has an analog output signal and this helps it in mesuring the concetration of carbonimonoxide in an enviroment 
this module opperates using 5V DC and has a low current consumption
 
Resources:
1. How to Calibrate & Use MQ9 Gas Sensor W/ Arduino
2. How to Interface MQ-9 Gas Sensor with Arduino