Unconventional biomasses as feedstocks for production of biofuels and succinic acid in a biorefinery concept

Abstract

Biorefinery has the potential of displacing fossil fuels and oil-refinery based products. Within the biorefinery a palette of marketable commodities can be produced from biomass, including food, feed, biochemicals and biofuels. Which bioproducts are produced is largely dependent on the chemical composition of the specific biomass feedstock, as well as which pretreatment, saccharification, fermentation and extraction techniques are used. Furthermore, integrating biological processes into the biorefinery that effectively consume CO2 will become increasingly important. Such process integration could significantly improve the sustainability indicators of the overall biorefinery process. In this study, unconventional lignocellulosic- and aquatic biomasses were investigated as biorefinery feedstocks. The studied biomasses were Jerusalem artichoke, industrial hemp and macroalgae species Laminaria digitata. The chemical composition of biomasses was determined in order to demonstrate their biorefinery potential. Bioethanol and biogas along with succinic acid production were the explored bioconversion routes, while potential production of other compounds was also investigated. Differences and changes in biomass composition and productivity of eleven different Jerusalem artichoke clones was examined at three harvest times. Yields of up to 35 t ha-1 of dry lignocellulose matter was reported, nonetheless the amount of cellulose in many cases was less than 50% of what was observed in e.g. hemp. However, the underground tubers which the plant produces, contained high amounts carbohydrates (≤88% of dry weight) and yielded up to 6 t ha-1 dry matter of additional carbohydrates. The carbohydrate content found in L. digitata was also shown to be exceptionally high (77.6% of dry weight) compared to other studies. Diverse methods for pretreatment and saccharification of biomass were used depending on the type of biomass. L. digitata did not required any pretreatment before enzymatic hydrolysis other than milling and drying. Pretreatments using H2SO4, NaOH and H2O2 at different conditions were used to pretreat hemp prior to enzymatic hydrolysis, while Jerusalem artichoke tubers needed 0.2% H2SO4 in combination with heat-treatment as a direct hydrolysis method. Bioethanol was produced from industrial hemp hydrolysates. Ethanol yields in the range of 74-92% of theoretical yield were reported, while ethanol concentrations amounted up to 10.0 g L-1. However, the production of succinic acid from this type of hydrolysate resulted in much higher product titer and substrate utilization compared to ethanol fermentation, partially because A. succinogenes is able to ferment both glucose and xylose into succinic acid. Jerusalem artichoke tubers, industrial hemp and L. digitata all showed considerable potential as feedstock for succinic acid production. The maximum succinic acid production from the different feedstocks ranged between 21.9 and 47.4 g L-1. The highest succinic acid titer was reached when fermenting Jerusalem artichoke hydrolysate, while the maximum succinic acid yield (86.5%) was reached when fermenting L. digitata hydrolysate. In the case of tuber biomass it was shown that tubers could be readily hydrolyzed without enzymes and fermented without any addition of nutrients, which clearly indicates that utilization of this feedstock could potentially lower the costs for succinic acid production. The biochemical methane potential of L. digitata, post hydrolysis solid residue (PHSR) and fermentation broth after succinic acid fermentation was also determined. In a biorefinery, biogas production is important for energy recovery as well as for minimizing waste and generating an additional product in the form of fertilizer. Energy recovery of PHSR and fermentation broth through anaerobic digestion corresponded to 298 and 285 NmL CH4 g-1 VSadded, respectively. To further increase the integration of the different processes in the biorefinery concept, a novel biogas upgrading technology was developed. The approach was based on the CO2 fixation abilities of A. succinogenes to simultaneously produce high purity CH4 and succinic acid. The system was able to reach 95.4% CH4 content, which is similar purity as commercial biogas upgrading technologies deliver. Results obtained in this study constitute the first report for utilization of macroalgae, hemp and Jerusalem artichoke tuber biomass for fermentative succinic acid production. It was demonstrated that all biomasses are attractive biomass feedstocks for succinic acid production mainly due to their high carbohydrate content. A case study of a proposed macroalgae biorefinery concept highlighted the potential of post hydrolysis solid residue (PHSR) for the production of numerous additional products such as ω-3 and ω-6 fatty acids, biodiesel, protein, feed, biogas and fertilizer, thereby diversifying the biorefinery product portfolio.