CO2 is one of the by-products of combustion. Any hydrocarbon fuel can be burned to produce CO2, except for those containing sulfur dioxide and ethylene which can be harmful to plants.
You can use dry ice. Two pounds of dry ice will raise the CO2 level in a 10-by-10 grow room to about 2000 ppm for 24 hours.
Fermentations is another way to add CO2 to small grow rooms if you can handle the smell. Use a one gallon plastic milk jug and mix one cup of sugar and a packet of brewers yeast with about three quarts of warm water. This concoction is changed one to four times daily as needed. Half of the solution is poured out, then 1 1/2 quarts of water and another cup of sugar are added. Add more yeast if needed.
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