Steam explosion is one of the hydrothermal treatments that subject the biomass to high-pressure saturated steam at the reaction temperature between 180 and 240 °C for several minutes followed by a rapid decompression. The residence time of steam treatment depends on the reactor type and the desired degree of treatment. The degree of steam treatment can be described by a severity equation developed by previous researchers [44]. This equation was developed based on modeling complex reaction systems by assuming that each reaction is homogenous, and Arrhenius dependence rate law and the temperature function were linearized by Taylor series [45,46]. This equation is developed based on the data from batch reactor.

where R0 = is the reaction severity, T is the reaction temperature (°C), t is the reaction time (min).

Another equation is also developed for scaling up from a batch process to a continuous process [47],

log R0,Batch = 1.50 X (log R0,Continuous!-)

where R,0,Batch is the reaction severity studied based on batch reactor, R,0,Contmuousis the reaction severity applied to the continuous large scale reactor.

chemistry

Under high pressure of saturated steam, the initial reaction that takes place inside the chemical components in presence of woody biomass is hydrolysis. Biomass is a mixture of polymer composite mainly made up of carbohydrates (cellulose, hemicelluloses) and lignin. In particular, hemicelluloses and lignin hydrolyze in the presence of acetic acid to release low molecular weight components: mono­sugars and acid-soluble lignin. Some mono-sugars will further degrade into other chemicals at high temperature by dehydration reaction. One typical example is the formation of furfural by dehydration reaction of xylose (mono-sugar of xylan, a type of hemicelluloses). For lignin, repolymerization or condensation reaction of low molecular weight lignin takes place as reaction coordinate proceed. This changes the structure and morphology of lignin, which is important for improving the binding ability of the wood fibers during pellet production. Detailed of chemistry has been reported previously [48].