Chemical pulp from organosolv digestion
The basis of the project is the physico‑chemical digestion of wood with alcohol‑water mixtures (organosolv process), making it possible to obtain all three main components of lignocellulose (i.e. cellulose, hemicellulose, and lignin). As an alternative to the production of glucose, the production of fibers and chemical pulp with this process is another economically viable option . However, the investigations by Peter and Höglinger contain considerable potential for optimization, especially with regard to the process conditions during pulping and bleaching. The quality and possible applications of lignin or hemicelluloses were also ignored. Therefore, the Fraunhofer Center for Chemical Biotechnological Processes CBP has adapted the pulping parameters to meet the necessary requirements for yield and purity of cellulose, but also of other products.
Chemical conversion of hemicellulose and lignin
The profitable value added from hemicellulose is still an important prerequisite for the economic viability of a lignocellulose biorefinery, in addition to the utilization of lignin. The German Biomass Research Center (DBFZ) was able to show that a conversion of hemicellulose to biogas is possible with high yields. However, higher‑quality material use is more economically viable. Hydrothermal processes are well-suited for conversion of aqueous systems (the hemicellulose fraction in this case), since water is simultaneously a solvent and a reaction partner in these processes [2, 3, 4]. The aim is to select the conditions so that sugars or furans can be selectively recovered from the hemicellulose fraction. In subsequent processing steps, they can then be separated as starting materials for fermentation or as basic chemicals.
The investigations on the reaction patterns of real hemicellulose fractions carried out at the DBFZ laid the foundations for scaling up to the mini‑plant scale at Fraunhofer CBP. In recent months, process windows were identified for the target products. To increase the value, the organosolv lignins obtained by fractionation is converted into phenol fractions by base‑catalyzed depolymerization, also under hydrothermal conditions. The process established at Fraunhofer CBP is further improved with regard to product selectivity as well as on energy- and material efficiency. In the future, a central issue will be further development of the separation and purification steps, in order to optimize the process with regard to technical feasibility, overall efficiency and profitability.
Fermentative production of malic acid and xylonic acid
In addition to the hydrothermal reaction, biocatalytic processes are also suitable for the conversion of pentoses and hexoses. There is a growing market for malic acid as it is used as a flavoring agent or for preservation of foodstuff. It also displays a potential as a polymer building block or in printing inks. The use of xylonic acid is also versatile, e.g. as a precursor for polyesters or hydrogels.
One task here is to investigate the usability of C5 sugars of the hemicellulose fraction. Fermentations for the production of xyllic and malic acid with xylose as substrate, which constitutes the main part of the C5 sugar in hemicellulose, are being developed at Fraunhofer IGB. In the second project phase, fermentation is carried out up to the 1‑m³‑scale in Leuna. Besides experiments in laboratory and pilot scale to optimize the fermentations, calculations for the scale up to the 1 m³ scale are carried out. During the scale up, geometric and material similarities should be ensured for the processes under consideration. In addition, the process conditions are to be transferred to the larger scale with the aid of mostly dimensionless scale-variant parameters such as kLa value or stirrer circumferential speed. These conditions are taken into account in the development of the new process and can be validated on a technical scale.
A further focus is on the subsequent processing of xylon and malic acid. After the biomass separation by separation/filtration a fine purification follows. Various methods and technologies, such as precipitation reactions and filtration techniques, are investigated. After detailed balancing and evaluation, a selected process is transferred to the 1 m³ bioreactor in order to demonstrate the technical feasibility of the entire process chain and to be able to provide products for application characterization in the polymer sector.
In addition to the economic assessment of the experiments carried out at Fraunhofer CBP, DBFZ will work out an ecological assessment of the advantages and disadvantages of this biorefinery concept in order to overcome the latter. Further pilot‑scale tests also enable the production of sample quantities for application characterization and determination of product specifications.