Pretreatment, can we achieve effective biomass hydrolysis without removing lignin?

A cost-effective pretreatment fractionates and recovers hemicellulose and lignin and produces a cellulosic fraction that can be readily hydrolysed at low enzyme loadings. Although hemicellulose can restrict access to cellulose, lignin typically has the greatest influence on the ease of cellulose hydrolysis. However, lignin is difficult and expensive to remove. Pretreatments based on chemical pulping such as organosolv remove lignin, but high capital and chemical costs have limited their commercialization. In contrast, mechanical pulping based processes, such as SPORL, typically have lower associated costs compared to the prevalent Kraft process. The waning demand for newsprint has encouraged some pulp and paper companies to assess the adaptability of refiner mechanical pulp (RMP) processes as a front end of a wood based biorefinery. However, lignin remains associated with cellulose after RMP treatment, possibly limiting its effectiveness as a pretreatment option. In the work that will be presented, RMP and steam pretreated poplar were compared to assess whether effective cellulose hydrolysis could be achieved without lignin removal. As previous work showed sulfonation post-treatment enhanced the ease of hydrolysis of RMP and steam pretreated poplar we assessed whether sulfonation could be integrated directly into each pretreatment process. This approach was highly effective, substantially reducing the enzyme loading for hydrolysis, combining the beneficial effects of pre/post treatment into one step. The likely mechanisms involved in this enhanced pretreatment/hydrolysis, (due to the higher reactivity of the less condensed lignin in the original biomass as compared to the pretreated lignin), will be discussed in more detail.