Lignocellulosic Paytocks: Pretreatment Analysis

As discussed in section 4.0, the biochemical conversion of lignocellulosic feedstocks to fuels and chemicals via soluble sugar intermediates requires a pretreatment step prior to the biological steps of enzymatic saccharification and fermentation (Mosier et al. 2005a). This pretreatment process helps to overcome the natural resistance of lignocellulosic feedstocks to biological degradation and reduces the recalcitrance of cellulose and hemicellulose to better enable conversion of these structural carbohydrates to soluble sugars (Elander et al., 2009). The pretreatment step is the most expensive capital investment in the biochemical conversion route of lignocellulosic feedstocks to ethanol (Wooley et al. 1999) and has been found to substantially

2005a). The increase of sugar yields by proper selection and optimization of the pretreatment operation can also impact the effectiveness and cost of the other process conversion steps. Pretreatment is found to directly affect the amount and types of enzymes required to further saccharify all plant cell wall structural carbohydrates to monomeric sugars, as some pretreatment approaches are able to hydrolyze virtually all of the hemicellulose directly to monomeric xylose while other pretreatment approaches largely leave the hemicellulose intact, or only achieve partial hydrolysis to oligomeric sugars (Mosier et al. 2005a). The type and severity of the pretreatment process can also affect the amount, type and concentration of potential inhibitory compounds that can be released in soluble form directly from the biomass itself (such as acetic acid from acetylated hemicellulose or phenolics or other organic compounds released from lignin) or those produced as products from sugar degradation reactions (such as aldehydes and organic acids most commonly associated with dilute acid pretreatment). These inhibitory products can affect the fermentation performance in subsequent steps and can require that hydrolyzate detoxification or conditioning operations be conducted, particularly from high solids pretreatments where the concentration of such compounds can be relatively