Understanding the roles of mineral matter in biomass processing to biofuels

FNU Fitria

doi:10.7273/000005073

Minerals in biomass have significant impacts on both biofuel quality and yield. This is especially true for current thermochemical biomass conversion processes. However, the roles of plant minerals in biochemical conversion have not been extensively studied, even though they are generally considered to lower the sugar yield due to reducing the carbohydrate proportion of feedstocks. Technologies need to be developed to overcome the following challenges due to the minerals in biomass, including 1) decreasing the quality of biomass feedstocks, 2) reduction of process efficiency, and 3) reduction of product qualities and quantities from biomass conversion. This thesis aims to understand the roles of minerals in biomass processing to biofuels, especially: 1) the influence of minerals in pretreatment and enzymatic hydrolysis processes; 2) the development of new solid acid catalysts from biomass ash and solid biomass residues for hydrolysis of biomass to sugars. The key contributions of this work include the following: 1. 30 PNW winter wheat varieties were examined to select appropriate genotypes to develop a dual-purpose crop (for grain and cellulosic ethanol production). Among the 30 tested wheat varieties, Weatherford ranked the highest for both straw yield and potential sugar production per land area and the 6th highest for grain yield. Based on this study, we recommend Weatherford as the best current variety for both grain and biofuel production. Further breeding efforts can be carried out to obtain wheat varieties with more promising performances as dual-purpose crops. 2. In this study, both high-ash (hypothetical, 20% addition of ash) and low-ash (without the addition of ash) corn stover samples were tested by using several current biomass pretreatment methods, i.e., liquid hot water, dilute acid, alkaline, tetrahydrofuran, γ-valerolactone, and ionic liquid pretreatments. Results showed that high ash content decreased sugar release from pretreatment and hindered the enzyme accessibility to cellulose during hydrolysis, leading to lower sugar yield. 3. A solution to build a closed-loop biorefinery by utilizing the minerals and solid waste from biorefinery can be an alternative option to increase biorefinery efficiency and economic value. This study used plant minerals and solid biorefinery residue to synthesize a sulfonated carbon-based solid acid catalyst. The catalyst without pre-carbonization before sulfonation yields high xylan monomers when used in xylan hydrolysis. When the pre-carbonization stage was conducted, the sulfonation was lower due to less binding sites available in the carbon, implying this additional step was not necessary. The impregnation of extracted metals from biomass ash enhanced the acidity of the sulfonated carbon-based catalyst (SAIW). The metals, especially aluminum (Al), acting as Lewis acid sites, simultaneously improved the acid site density. The optimum catalysts (SAIW at 200 wt% catalyst loading) resulted in 72.87 g per 100 g initial solid of xylose monomer recovery, comparable to dilute acid hydrolysis (74.09 g per 100 g initial solid) under the same reaction conditions. At the same time, SAIW significantly inhibits the formation of furfural. The catalyst loading comparisons show no significant difference in xylose monomers yield at 200 and 300 wt% catalyst loadings (2 g catalyst/g xylan and 3 g catalyst/g xylan, respectively), while the 300 wt% loading favors the secondary dehydration reaction to produce furfural. This study shows the potential application of biorefinery waste-based solid acid to replace dilute acid in biomass pretreatment, making use of the waste from the biorefinery, supporting the concept of a closed-loop biorefinery. Further research is recommended to improve metal extraction and impregnation from biomass ash, catalyst loading reduction, and catalyst reusability. The findings can inspire better strategies to deal with the variability of mineral content in biomass feedstocks to increase process efficiency and reduce the cost while supporting the concept of a circular bioeconomy.

Understanding the roles of mineral matter in biomass processing to biofuels

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