Limitations of Conventional Pretreatment Technologies
Various chemical pretreatments that are commonly applied to overcome the recalcitrance of lignocellulosic biomass modify the lignin, resulting in nonproductive bindings with the enzymes, thus lowering the rate of enzymatic hydrolysis. At the same time, the lignin becomes either condensed or contaminated with the chemicals. This results in poor sugar yields from woody biomass especially because of the higher lignin content present as compared to agricultural residues or herbaceous crops.
Other significant limitations of these chemical pretreatments are the higher capital and operating costs associated with them as discussed below:
Dilute acid pretreatment is operated at lower pH conditions which require that the pretreatment reactor and the associated piping be constructed with higher-grade steel increasing the capital cost of the pretreatment. AFEX pretreatment uses expensive ammonia which requires that sophisticated recovery and recycling to be in place which increases the capital cost. It is also important to consider the health hazards associated with ammonia which require that more stringent operator training and regular process hazard analysis has to be documented which increases the operating cost. Sulfite pretreatment also requires organophilic solvents to remove the sulfonate groups that get attached to the lignin and thus adds further process equipment for the recovery of these solvents, leading to increased capital cost.
Considering the fact that pretreatment is the first and very expensive step in the biomass to biofuel conversion process, it is essential to not only produce higher sugar concentrations to get the maximum value, but also it should not add excessive process equipment and cost. It is also important to understand that softwood contains highest amount of lignin proportion among lignocellulosic biomass and this lignin fraction should not be neglected, rather it should be considered for the conversion into useful chemicals or intermediates.