CO2 is 72.7% oxygen by weight, and 27.3% carbon. 2kg is 28.5% of 7kg, so with rounding it works out even if that is assuming that the coal has no impurities - it may be closer to 6kg but essentially it takes oxygen from the air and adds it to the carbon in the coal to form CO2
Am I missing something? How does 2 kg of coal release 7 kg of CO2?
CO2 is 72.7% oxygen by weight, and 27.3% carbon. 2kg is 28.5% of 7kg, so with rounding it works out even if that is assuming that the coal has no impurities - it may be closer to 6kg but essentially it takes oxygen from the air and adds it to the carbon in the coal to form CO2