The most investigated pretreatment method is steam explosion (or steam pretreatment) with and without addition
of acid catalyst. Of the acids, H2SO4 has been most
extensively studied, apparently because it is inexpensive and effective. SO2- catalysed steam pretreatment has also been thoroughly investigated . Pretreatment using gaseous SO2 does not induce corrosion in the same manner as does
pretreatment with H2SO4, and it is also easier and faster
to introduce into the material. The main drawback of SO2 is its high toxicity, which may pose safety and health risks. However, SO2 is used in various industrial processes using established techniques. Other pretreatment methods include alkaline and solvent pretreatment techniques, which are basically delignification processes