Collaborative Research: Elucidation and Evaluation of Strategies to Mitigate Secondary Reactions in Cellulose Pyrolysis for Enhanced Production of Hydrolysable Anhydrosugars

The fast pyrolysis of cellulose under deep vacuum results in the production of more than 95 wt. % hydrolysable sugars that could be used for various purposes; however, the yields of sugars (mainly levoglucosan) obtained in most pyrolysis reactors processing lignocellulosic materials rarely exceed 10-20 wt. %. Various strategies including the application of vacuum, feedstock acid washing and/or acid addition produce levoglucosan yields close to 60 wt. % from lignocellulosic material (on cellulose basis). The literature and our preliminary results suggest that the decrease in the yield of pyrolytic sugars in acid washed materials is due to poorly understood secondary reactions. These reactions are caused by the low volatility of the primary depolymerization products from cellulose (mainly oligo-anhydrosugars). The secondary reactions cause excess char formation as the primary products undergo secondary dehydration, crosslinking, and polycondensation reactions in a liquid intermediate phase inherent to pyrolysis. Another phenomenon affecting the yield of sugars is the chemical interaction of cellulose with other components in the lignocellulosic matrix; this can be mitigated with the use of acid additives. Cellulose depolymerization reactions have been extensively studied, and it is also well known that the presence of alkaline metals decreases sugar yields. However, there are very few publications regarding the secondary reactions such as dehydration and crosslinking that are responsible for the reduction in the yield of anhydrosugars in acid washed materials and materials with acid added before pyrolysis. The main practical goal of this proposal is to mitigate the secondary reactions to maximize the yields of fermentable anhydrosugars close to those achievable by primary reactions (95%). In this proposal, we will use experimental and modeling tools to better understand (1) the secondary reactions in the intermediate liquid-phase sugars in cellulose pyrolysis and (2) the mechanisms by which acids mitigate the interactions of cellulose (or its primary products) with the lignocellulosic matrix (or their products). Micro-explosion enhancers will be evaluated to promote anhydrosugar aerosolization for a lower residence time in the pyrolytic liquid intermediate and a decreased probability for undesirable secondary reactions inside the pyrolysis reactor. Intellectual merit of the proposed activity: This proposal, for the first time, will combine experimental and modeling tools to better understand the role of liquid phase oligomeric anhydrosugars (cellobiosan, cellotriosan,…) in the undesirable dehydration, cross-linking, and polycondensation reactions. The mitigation of very poorly understood interactions between cellulose (or its primary products) and the lignocellulosic matrix (or the products of lignin or hemicelluloses) in acid washed lignocellulosic materials will be investigated. A new strategy using additives to enhance the microexplosion of liquid intermediates will be studied as a way to form aerosols for the recovery of heavy sugar products. Broader impact: We will broaden our impact by working with Walla-Walla Community College (WWCC) in the development and evaluation of courses for associate degrees in Bioenergy Operations and in Renewable Resource Recovery Operations. We will organize an annual workshop between our graduate students and postdoctoral researchers and WWCC students to exchange ideas and encourage WWCC students to continue their professional development. This project will also serve as a platform for under