Review of Biomass Pretreatment Methods for Biofuel Production: "Fundamentals Toward Application"

A recent review by Valery Agbor and colleagues from the University of Manitoba (Canada) provides an overview of the methods (and latest trends) being used in the pretreatment of lignocellulosic biomass for the production of "second generation" biofuel-ethanol. Pretreatment is the first of three steps in the conversion of lignocellulosic biomass to ethanol. The next two sequential steps are saccharification and fermentation. The main objective of pretreatment is to "deconstruct" the plant cell walls, by destroying the tight lignin wrapping surrounding the biomass, resulting in the exposure of the carbohydrate polymers (mainly cellulose). The exposed carbohydrate polymers can then be easily converted to simple sugars in the second-stage saccharification step,and these sugars can then be fermented to ethanol. 

The pretreatment technologies can be broadly classified as: (1) physical, (2) chemical, (3) biological, and (4) multiple/combinatorial treatments. Physical pretreatments include: comminution (grinding to particle size) and gamma-ray treatment. Chemical pretreatment methods include the use of acids, alkali, organic acids and ionic liquids. Biological methods involve the use of microorganisms (usually fungi) for the degradation of lignin and hemicellulose. Combinatorial pretreatments (usually physical plus chemical) include: steam(explosion)treatment,  the use of liquid hot water (170^oC to 230^oC), treatment with dilute acid (less than 4% by weight) plus heat, ammonia fiber/freeze explosion (AFEX), lime plus wet oxidation pretreatment, and organosolve pretreatment. A brief process description is presented for each method, followed by advantages/disadvantages, and the potential for large-scale application.

The review concludes that biomass pretreatment  "remains a key bottleneck in the bioprocessing of lignocellulosics for biofuels and other bioproducts." Also, the diversity of biofuel feedstocks makes it unlikely, that "one method will become the method of choice for all biomass" feedstocks. The full review is published in the journal, Biotechnology Advances.