The chemical energy trapped by photosynthesis is stored in sugars. It is found mostly in grains. Hemicellulose is a structural polymer made out of xylan, glucose, mannan and other components. Lignin is a polymer made up of phenyl propane units. Lignin is more nearly an aromatic material, but it has many side chains and cross linkages.
Angiosperms tend to have less lignin and more hemicellulose than gymnosperms.
Most agricultural residues are angiosperms. Starchy materials found in grains are easiest to use. Because starch is an energy storage polymer for the plant, it is biochemically designed to come apart easier than cellulose or hemicellulose. Also the glucose in starch is easier to ferment than the hemicellulosic sugars.