Any biomass is characterized by shape, size, moisture content and ash content. Shape and size do not matter because you need to pulverize the material (s) into relatively fine size - typically 1 to 2 mm and then mix them and briquette the materials. Moisture has to be reduced to a minimum to enable operation and ensure that the briquettes have enough energy. Woody biomass has less than 1 % ash fraction and grasses about 10 to 20 % (rice husk and rice straw have 16 to 20 % ash. This reduces the calorific value significantly. What gets used depends on the availability of the residues. All agricultural residues are seasonal and they also have multiple uses, the most important being fodder for domestic animals.