If you want to find out the amount of carbon in a biomass, one standard method is known as Ultimate Analysis. Another standard method is Proximate Analysis. You can explore further in the internet based on the two laboratory analyses. All the best.
You must have an ultimate analyzer to determine the Ultimate analysis or the carbon content.
In the case of proximate analysis, you simply require a muffle furnace.
Although, the ultimate analysis will give you accurate and direct results than the proximate analysis. If you need additional methodology, just put it here.
Washing and drying, carbonization and activation using chemical or physical methods, and thermal treatment in an inert environment at high temperatures are usually involved in the preparation of carbon from biomass waste.