Enzymatic hydrolysis is a critical process in the conversion of lignocellulosic biomass into biofuels and biochemicals. Achieving high efficiencies and productivity during the enzymatic hydrolysis of biomass is the key for the commercially viable process. Downstream processing challenges require high product titers which in turn require the use of high solid concentrations during enzymatic hydrolysis. Challenges such as low hydrolysis efficiency, high energy consumption, poor mixing quality and high maintenance requirements at high solids concentrations necessitate operational strategies and the system design-based solutions.
This project aims to develop a system capable of processing high solids content slurry and identify the strategies for high products concentration while maintaining low energy consumption.
Fed-batch approach was used to successfully demonstrate high glucose and ethanol concentrations after hydrolysis and fermentation respectively. With 45% (w/w) solids loading of corn stover, the released glucose concentration was 205 ± 25.8 g/L at 96 hours, while ethanol concentration was 115.9 ± 6.7 g/L at 156 hours.
Various surfactant concentrations were evaluated to determine their effectiveness. The experiments were conducted in the 0–2.5% for PEG6000 using 30% solids loading of wheat straw using separate hydrolysis and fermentation (SHF) and simultaneous saccharification and fermentation (SSF).
The synergetic effect of combining the fed-batch method with surfactant addition was investigated. Various surfactant concentrations were evaluated to determine their effectiveness. The experiments were conducted in the 0–2.5% for PEG6000 using 30% solids loading of wheat straw using separate hydrolysis and fermentation (SHF) and simultaneous saccharification and fermentation (SSF). The results illustrated a significant improvement in the final glucose and ethanol titers when PEG6000 was used. However, a detailed economic analysis of the various process options indicated that the PEG concentrations <1% are preferred when using return on investment as a performance criterion.
Reactor design and configuration were comprehensively tested in this work. Horizontal reactors represent a potential solution to some of the challenges due to the ability to provide high mixing quality for high solids enzymatic hydrolysis (HSEH) with lower mixing energy requirements compared to vertical reactors. A system consisting of a horizontal reactor with a novel design of impeller that integrates the functions of helical impeller and paddlewheel was constructed. A feeding unit was built and installed on the system to control the biomass addition into the reactor. The system demonstrated its superior performance at high solids loading (40%) as measured by the final glucose and ethanol concentrations. Combining the horizontal reactor system with the surfactant (PEG 6000) addition at 0% and 1% concentrations, the glucose concentrations were 201.4 g/L and 219.7 g/L respectively. Ethanol concentrations during the SSF were 134.5 g/L with the addition of 1% PEG6000. The feeding unit was well controlled and was able to provide the required amount of biomass. Furthermore, the system was able to maintain a low level of energy consumption at 43.2 Wh/kg. Based on these results, the fed-batch approach for the SSF method with a 1%PEG 6000 is the recommended strategy for operating the novel horizontal system.
To further evaluate the system performance from economic and environmental impact perspectives, a detailed techno-economic analysis and life cycle assessment were performed. The results of the techno-economic analysis indicated a return on investment (ROI) of 12.21% when operating the system using the best scenario (fed-batch, SSF, 1% PEG600, and 72 hours). The sensitivity analysis indicated that the selling price of ethanol is the most important factor confirming the results observed by other researchers. The biomass price and plant production capacity were the next two most important factors for economic viability. The LCA results indicated that the system has lower environmental impacts in many impact categories such as GWP, acidification, ecotoxicity, and eutrophication, in addition to human health.
This research, at a fundamental level, developed technologies in the areas of biofuels and biochemicals by developing controllable reactor systems that address some of the challenges in the hydrolysis and fermentation of biomass at high solid concentrations. Based on the experimental results, the techno-economic and life cycle analysis, the proposed system design and operational strategies were found to be technically feasible and economically viable with lower impacts on the environment compared to the state-of-the-art technologies.