Rotating reactor used at the USDA Forest Products Laboratory
Rotating reactor used at the USDA Forest Products Laboratory

NARA researcher J.Y. Zhu and his team are trying to solve a problem. They are refining a pretreatment method used to soften the wood and allow the simple sugars to be released. It is the simple sugars that are used to make alcohols, which are the building blocks to produce bio-jet fuel. Removing simple sugars from wood requires some pretty harsh treatments. Too harsh of conditions will result in altering the simple sugars into unwanted chemicals collectively called inhibitors.  In this case, not only are the sugar molecules degraded, but the inhibitors created interfere with downstream processing steps necessary to ferment the sugars into alcohols and generate co-products. As the severity of the pretreatment is reduced, however, fewer simple sugars are released which decreases yield.

In a recently published paper funded by NARA, J.Y Zhu and his team evaluate how pretreatment temperature affects simple sugar yield and sugar degradation into inhibitors. They evaluate the inhibitor formation and simple sugar yield using two temperatures (165° and 180° C) for the SPORL (Sulfite Pretreatment to Overcome the Recalcitrance of Lignocellulose) pretreatment method. This pretreatment method relies on heat, chemicals and time, so to make up for the decrease in heat, they assigned a reaction time of 75 minutes for the 165°C temperature condition and 30 minutes for the 180°C reaction. By adjusting the reaction times, the “reaction severity” stayed relatively equal between the two conditions.

To read “Using low temperature to balance enzymatic saccharification and furan formation during SPORL pretreatment of Douglas–fir” click here.

Lower heat reduces inhibitor formation

Identifying high temperature as a cause for inhibitor formation was based on the theory that sugar degradation into inhibitors from hemicellulose requires more energy (think heat) than the heat necessary to allow for effective simple sugar yield. As it turned out, the pretreatment set at 180°C at 30 minutes produced approximately double the concentration of inhibitors as the conditions at 165°C for 75 minutes, yet the sugar yield efficiency for both conditions was the same. In addition, the change of temperature did not have a significant affect on lignin removal. Designing an efficient pretreatment option with lower temperature not only helps reduce inhibitor formation, but will also allow for greater flexibility in using existing infrastructure that can handle the lower temperatures in the pretreatment process.

Background information regarding pretreatment

Carbohydrates, often referred to as monosaccharides, or in laymen terms, simple sugars, are the molecules from wood that are used to make bio-jet fuel. Removing intact simple sugars from the wood material is a challenge because first, they are tightly bound together in the form of polymers called cellulose and hemicellulose and because second, these polymers are surrounded and protected by a complex molecule called lignin. The combined structure containing cellulose, lignin and hemicellulose is called lignocellulose.

Removing the simple sugars from lignocellulose involves two steps. The first step is called pretreatment which separates the lignin, hemicellulose and cellulose. The second step is enzymatic hydrolysis, in which enzymes, that break the bonds in cellulose and hemicellulose, are added to the pretreated wood and thus release the simple sugars.

To see where pretreatment and enzyme hydrolysis occur in the NARA supply chain click here.

There are a number of methods used for pretreatment. These methods rely on an input of energy and/or chemicals to disrupt the interaction of lignin, hemicellulose and cellulose.

Read here for a description of various pretreatment technologies evaluated by NARA

NARA will select one pretreatment process

NARA has evaluated four pretreatment options and will decide on a single option within the next month. Some parameters that affect the choice of pretreatment options will be sugar yield, inhibitor formation, scalability, capital and operations costs, air emissions, and adaptation to existing facilities. Selecting a single pretreatment option is a significant NARA milestone and allows NARA researchers to create a comprehensive techno-economic analysis and a life cycle assessment that is based on a single conversion technology.