Date of Award
Master of Science
Robert C. Brown
In the search for a renewable energy source that could replace petroleum and other nonrenewable energy sources, pyrolysis of biomass is a hopeful alternative and pretreatment could further improve its potential. There are several possible routes for pretreating lignocellulosic biomass, but many need further refining before becoming economical. The main obstacle for the conversion of biomass to high quality products is the presence of alkali and alkaline earth metals (AAEMs) that are known to impair cellulose decomposition to levoglucosan and promote char, water, and light oxygenate formation. One pretreatment option is to remove the AAEMs from the biomass by washing. Another efficient process of converting biomass into high quality bio-oil is a dilute acid pretreatment that passivates AAEMs in biomass, leading to decreased cellulose monomer fragmentation and thus increasing sugar yields. In this study the effectiveness of a washing method--involving a recycled carboxylic acid rich, aqueous fraction of bio-oil--is looked at for red oak. This study also presents an optimization of three important variables in the dilute acid pretreatment process of the agricultural byproduct, cornstover, to increase the value of the bio-oil produced.
In the preliminary study, red oak was washed with an aqueous fraction of bio-oil to remove the AAEMs. While effective at removing AAEMs, the levoglucosan yield did not improve, unless a water rinse was incorporated--increasing levoglucosan yields from 3.3 to 13.3 wt%. Passivating the remaining AAEMs in the washed samples with sulfuric acid was less effective but increased levoglucosan yields from 3.3 wt% to 9.8 wt%. These processes were compared to samples that were only passivated with sulfuric acid, which led to levoglucosan yields of 20.8 wt%.
In the pretreatment optimization study for cornstover, the variables considered were the particle size upon acid infusion, the biomass to water ratio, and the diffusion time. Pretreatment of 400 g batches of cornstover was carried out in a paddle mixer equipped with a pump and sprayer. From these batches 250 μg samples were pyrolyzed to examine their effect on product yields, mainly levoglucosan. Levoglucosan yields increased more than 2,000%, reaching yields as high as 17 wt% on a dry biomass basis. With process optimization the amount of water necessary for acid passivation can be reduced significantly. Reducing the amount of water in the pretreatment process led to increased levoglucosan yields; a 2:1 biomass to water ratio was found to be more effective than a 1:1 ratio at increasing yields. The particle size also played an important role. 3.17 mm particles resulted in the highest levoglucosan yields. Diffusion time was not an important factor in the acid infusion process. Overall, optimal levoglucosan yields were achieved with 3.17 mm cornstover particles, a 2:1 biomass to water ratio, over any length of diffusion time.
Kayla Elizabeth Johnson
Johnson, Kayla Elizabeth, "Pretreatment optimization methods for increased sugar yields from biomass pyrolysis" (2017). Graduate Theses and Dissertations. 15328.