Recently proposed various solid biomass fuels are reviewed from a safety aspect, especially concerning the hazard of spontaneous ignition. Various types of biomass materials are proposed for use as fuel, such as RDF (refuse derived fuel) and wood chips. These fuels have high energy and potential to cause fires, explosions, or other incidents. Japan experienced many incidents with biomass materials, such as wood chips, coal-wood mixture, and organic rubble produced from destroyed houses by the great earthquake and tsunami in March 2011, in Japan. Organic rubble includes various organic materials and sometimes causes fire, and it may be able to be used for fuel of power plants. However, it is very difficult to extinguish fires of biomass fuel piles in storage facilities. Here, current studies on heat generation for these materials and a proposed evaluation test method for these new developing materials in Japan are introduced, which is to use high sensitivity calorimeters such as C80, or TAM, and gas emission test. Additionally, cause investigation work for post-earthquake fires by organic rubble, conducted by the author, is introduced. High sensitivity calorimeters can detect small heat generation between room temperature and 80 degrees C, due to fermentation or other causes. This heat generation sometimes initiated a real fire and produced some combustible gas, which may explode if fuel is stored inside silo or other indoor storage.