Researchers: David Nickel, Björn Alriksson and Emma Johansson (SP Technical Research Institute of Sweden), Ruifei Wang, Lisbeth Olsson, Carl Johan Franzén
In second generation bioethanol production it is often hard to reach high ethanol concentrations in the fermentation without concentrating the sugars released from hydrolysis of the lignocellulosic material. The material contains water insoluble substances and therefore it is difficult to stir at the high solids loadings that are required to reach high ethanol concentrations. Furthermore, the high solids loadings also leads to high concentrations of inhibitors, that slows down or even stops the fermentation process. High ethanol concentration reduces the energy requirement for distillation of the ethanol and for drying of the stillage, which leads to sustainable bioethanol production.
In the multi-feed SSF approach we address these issues by controlled feeding of solid materials, yeast cells and enzymes. The solid materials are added based on pre-determined kinetics for the enzymatic hydrolysis of the added materials. In addition, by feeding fresh yeast cells, that have been adapted to the actual material during the process, the fermentation rate is kept high and high final ethanol concentrations can be reached.
We use mathematical models for enzymatic hydrolysis and for fermentation to optimise the process. With the models, we will develops a framework based on sub-models of the bioethanol process to reveal the impact of raw materials on cell and process conditions. The developed sub-process models will be used for optimization and control of SSCF processes based on properties of the raw materials. We have so far worked with spruce chips, birch chips and wheat straw, and extend now with spruce residues (branches and tips) and oat husks.
The multi-feed SSF process for 2nd generation bioethanol production from renewable raw materials, e.g. wheat straw, has been improved dramatically by continuous development in laboratory scale. We have obtained final ethanol concentrations of 65 g/L with yields about 70% of the theoretical based on total sugar input. To explore the potential of our process at a scale closer to real application, we also study how the process performs at the 10 m3 scale at the Biorefinery Demo Plant in Örnsköldsvik, Sweden. Process integration aspects are studied in a dedicated parallel project.
We expect that the process will be applicable to biotechnological production also of other chemicals by enzymatic hydrolysis and fermentation lignocellulosic materials
This research is funded by the Swedish Energy Agency and the Chalmers Energy Initiative