Lignocellulose biorefinery

Sustainable use of renewable resources

Rising prices for raw materials and the increasing scarcity of mineral oil resources are boosting the interest in renewable resources and in strategies for the sustainable production of special materials, fine chemicals or platform chemicals. Emulating the concept from the petrochemical industry these biobased materials will in future be made available by biorefineries. Native hardwoods offer themselves as a chemical feedstock that is available sustainably and without competing with foodstuff production. For the conversion of wood or lignocellulose into platform chemicals for the chemical industry of the future, the wood first of all has to be broken down and separated into its basic chemical components.

Organosolv lignin for a wide range of applications

In the second phase of the collaborative project “Lignocellulose Biorefinery” the concept of a wood‑based biorefinery was successfully implemented on the pilot scale at the Fraunhofer CBP in Leuna. The necessary preliminary work for this was carried out together with the Fraunhofer Institutes IGB and ICT as well as twelve other industrial and research project partners. The “organosolv process” developed in the first phase of the project uses mixtures of alcohol and water to fractionate wood under pressure and temperature into its basic components cellulose, hemicelluloses and lignin. Cellulose and hemicelluloses can then be converted to sugars with the help of enzymes. Unlike with other breakdown processes, the resulting lignin is very pure and neither contaminated with sulfur nor with inorganic salts. This permits applications for a wide range of materials.

Scaling of the fractionation process to the pilot plant

The work at the Fraunhofer CBP focused on the scaling of the fractionation process to the pilot scale. The basic questions concerning the process specifications were worked on in joint preliminary work with the project partners. The plant was designed and planned in detail on the basis of these results.

The pilot plant comprises a large number of individual process steps for the production of concentrated sugar solutions and lignin powder obtained from wood chips. Up to 70 kg of wood (oven dry weight) can be processed in one batch. The plant was designed in such a way that the material and energy cycles are closed. The simplified diagram of the plant in Fig. 2 shows that the wood is first of all digested in a 400‑liter reactor at up to 200 °C; here lignin and hemicelluloses are solubilized in an ethanol‑water mixture. The additional tanks and heat exchangers of the “tank farm” permit a fractional washing of the material under reaction conditions and the recovery of energy during the pulping process. From the pulping liquor enriched with lignin and hemicelluloses lignin is precipitated by adding water or distilling the ethanol; it is then filtered off and, after washing, dried. The ethanol used is entirely recovered by distillation from the filtrate and the hemicellulose sugars remain as an aqueous solution. The solid fibrous residue of the pulping process is disintegrated and washed, dewatered, enzymes are added and then, in stirred reactors specially designed at the Fraunhofer IGB, it is saccharified with a high concentration of the pulp. After a filtration step a glucose solution is obtained which is concentrated into a sirup to stabilize it.

The pilot plant was built at the same time as the CBP was constructed. The pilot plant was successfully put into operation in spring 2013. Since then we have been further optimizing the individual processes as well as the overall process. Additionally, complete mass balances were recorded and product samples on the kilogram scale were distributed to the project partners so that they could carry out technical application tests.

Basic feedstocks for chemical and material utilization

The intermediate products obtained in the pilot plant – lignin and sugar – serve as feedstocks for utilization as chemical substances and other materials. The sugars achieved from the wood were used by the project partners as a feedstock for industrial biotechnology processes, in order to produce for example basic chemicals such as lactic acid, acetic acid, succinic acid or ethanol. The advantage of the sugars extracted from wood is that they do not compete with food production.

Because of its good thermoplastic properties the lignin obtained was used directly by the project partners in compounds for the extrusion of molded parts or as an alternative for the mineral oil‑based phenol in resins and polyurethane compounds. The project partners also examined various chemical and biotechnological possibilities for lignin cleavage to create basic aromatic constituents. Altogether a large number of processes and products were developed with the sugar and lignin samples from the pilot plant.

Outlook

With the successful transfer of the organosolv fractionation process to the pilot scale we were able to demonstrate that the chemical utilization of wood using a biorefinery concept works on the technical scale. The pilot plant is being used within  various international and national research projects to further optimize the process, to integrate the intermediate products obtained in various value chains and finally to pave the way for the industrial implementation of lignocellulose biorefineries.