Hi Raju,
Somany different CO2 sensor modules are available, most of them gives analog output voltage ranging from 0 to 4 volts depending upon the concentration of CO2 gas.
Simply connect the module to your ADC, read the value and compare it with a threshold value of detection.
Find the following link and the similar projects from this site to learn more about how to use ADC with 8051 and please feel free to post your further queries.
engineersgarage.com/microcontroller/8051projects/interface-ADC0804-AT89C51-circuit