Pretreatment of Tropical Lignocellulosic Biomass for Industrial Biofuel Production : A Review

As energy demand increase with human activities, the practical solution this far is to use more fossil-based energy to meet those demand. Diminishing fossil energy reserves that can cause crisis and also the increasing price of fossil energy are the main challenges for the world, and are a big problem for developing countries. Environmental factors, sustainability, and national resilience are the main considerations for replacing fossil energy with non-fossil alternative energy such as solar energy, hydro energy, wind energy and biomass or biofuel energy. Lignocellulosic biomass which is a waste from the agricultural industry, livestock industry waste, solid waste and other materials is the most abundant source of biomass that can be used to produce biofuels. In Indonesia, the most abundant sources are palm oil empty fruit bunches, corn stover, rice husks, rice straw, sugarcane bagasse, and coconut husk. Utilization of lignocellulosic biomass to be converted into energy requires pretreatment to change interactions between components found in plant cell walls. Pretreatment can remove physical and chemical barriers that make lignocellulosic material difficult to react and difficult to access by enzymes for the hydrolysis process. While the hydrolysis process is a vital stage in the biochemical process from lignocellulose to sugar-based energy conversion. By comparing several pretreatments based on glucose yield and other technical factors, it can be concluded that steam explosion is the best pretreatment by the reason of not requiring a small sizing on the biomass so that it is more energy efficient; not use chemicals; low water content causes energy saving; no degradation of cellulose and lignin so that sugar yields are higher and lignin can be recovered; and also the results of hemicellulose degradation can be separated by leaching and having economical value. The mechanism of the process that occurs in the steam explosion pretreatment is that high temperatures cause the release of acetyl groups on the hemicellulose structure which causes autohydrolysis to degrade hemicellulose; high temperatures cause steam to be acidic and degrade hemicellulose and hydrolyze some cellulose; sudden pressure drop causes redistribution of the lignin structure so that the lignin structure is not damaged and can be recovered; the loss of lignin and hemicellulose which blocks enzyme access to cellulose causes an increase in glucose yield during the enzymatic hydrolysis process. The steam explosion pretreatment method is not effective for hard biomass, but that raw material is more effectively processed using the alkaline method.