used the gas sensor MQ21 with Arduino UNO
The MQ series of gas sensors use a small heater inside with an electro-chemical sensor. They are sensitive for a range of gasses and are used indoors at room temperature. The output is an analog signal and can be read with an analog input of the Arduino. The MQ-2 Gas Sensor module is useful for gas leakage detecting in home and industry. It can detect LPG, i-butane, propane, methane ,alcohol, hydrogen and smoke. Some modules have a built-in variable resistor to adjust the sensitivity of the sensor.
Note: The sensor becomes very hot after a while, don't touch it!