It seems that the MQ-7 has to be driven with alternating voltages. I don't see how the lm393 should do that - and there is not much else on the board.
Also I wonder if these things are trustworthy. I just got a Nano with forged USB-to-Serial chip that works with Linux but not with Windows 7, so I became a bit careful. I don't really see a way to test them.
On the other hand, it is really just for the extra safety. I want to put it next to a well built, externally ventilated and officially tested fireplace, so there is not too much to worry about.
I've not used those sensors but according to Sparkfun
The sensor's output is an analog resistance. The drive circuit is very simple; all you need to do is power the heater coil with 5V, add a load resistance, and connect the output to an ADC
I am a little suspicious, since I see no way to test this thing. Perhaps I could burn a candle or wood in a closed jar and see if something happens - At least, that is the worst thing that could happen in my fireplace in case the external ventilation and/or exhaust fails somehow.