Things don't all burn up (the combustion is incomplete) when there is not enough oxygen available to convert all of the carbon in a sample into CO2. Consider the combustion of methane (CH4):
CH4 (g)  +   2 O2 (g) CO2 (g)  +   2 H2O (g)
The balanced reaction tells us that in order to completely burn all the carbon in one mole of methane and make CO2 we need 2 moles of oxygen. Or, we need 32 grams of O2 for every 16 grams of CH4. If there is less than 32 grams of O2 for every 16 grams of CH4, we will produce some CO.
The problem is that CO is a toxic gas and even worse, it's colorless and odorless.
Fortunately, we have chemical methods that allow us to detect CO. Here's how they work.
So, what are the results of incomplete combustion? What's the black solid stuff you see left over after many fires? What's the black stuff you see in the smoke? Think about it. You can figure it out!
Now let's continue on and look at the combustion products of cellulose nitrate.