Citric acid can actually exist in two different powdered forms, depending on how it is prepared- either anhydrous (without water) or mono-hydrate (with some amount of water trapped in the powder with the citric acid molecules).

Citric acid is a weak, carboxylate acid. If you heat powdered citric acid to its melting temperature of 153degrees Celsius / 307 degrees Fahrenheit, then the powder will liquefy into a clear or brownish liquid (as additional heat is absorbed) and will burn. If you heat it slightly hotter, to 175 degrees Celsius / 350 degrees Fahrenheit, which is what a sustained fire would do, it will spontaneously decompose --that is, break down into smaller molecules including water and carbon dioxide (CO₂). These smaller molecules will be very hot, and will become gasses- steam and CO₂ in the air, which means that it will expand very quickly, i.e. it may explode. Be careful!

Citric acid is an organic compound; it will burn. I do not know how, specifically, it will combust, or if it will combust completely or not at moderately high heat, but it burns.

Citric acid is an organic compound made up of carbon, hydrogen and oxygen, so it should burn like any other organic substance. Please be careful.