Biomass energy uses organic matter such as wood or plants - lignocellulosic biomass - for creating heat, generating electricity and producing green oil for cars. Modem biomass energy recycles organic leftovers from forestry and agriculture, like corn stovers, rice husks, wood waste and pressed sugar cane, or uses special, fast-growing "energy crops" like willow and switchgrass, as fuel. Biomass is composed of three major components: cellulose, hemicelluloses, and lignin. Their differences in chemical structures lead to different chemical reactivities, making the relative composition in cellulose, hemicelluloses and lignin in the biomass a crucial factor for process design. In this paper thermogravimetric analysis is investigated as a new method to obtain lignin, hemicellulose and alpha-cellulose contents in biomass. It is shown that this alternative method lead to comparable results than common methods used for the determination of the alpha-cellulose content, with an enhancement of the accuracy in the determination of the hemicellulose content. Unfortunately, this method cannot be adopted for the determination of the lignin amount.