(6e) Organosolv Revisited – Biomass Valorization with New Perspectives

Due to fading fossil resources a strong need for valorization of renewable resources is on the rise. Bioconversion of renewable resources by treatment with hot aqueous ethanol, known as the Organosolv process, is an old concept dating back to the early 1950s. With the beginning of the 1980s the interest in this research area increased remarkable focusing on the delignification of renewable materials, mainly wood. With the establishment as a promising approach for the future, new possible facets appeared in scientific databases. Organosolv pulping as a conceivable „green“ alternative for established sulfur-containing processes as kraft and sulfite, generating enormous amounts of non-recyclable by-products, gained widespread interest with its peak in the nineties. In the following years the focus was directed to other possible renewable raw materials for Organosolv processes like straw and annual plants. The latest development in this area the use of Organosolv principles as a pretreatment step for various renewable resources to enhance the bioethanol output from polysaccharides in bacterial digestion.      

The everlasting issue in energy production based on renewable materials is how to make such a process profitable. Resources like straw are cheap but the energy input for the transformation to biogas is high rendering a simple conversion less feasible. As pulp resulting from Organosolv treatment of straw and other annual plants does not show sufficient quality to access the paper and fiber industry, other main constituents need to be transformed to value-added products to sustain such a concept. Lignin, one of the main components in all naturally grown materials, shows a high potential to fulfill this demand although efficient usage could not be established in the past. Until now rather neglected by simply using it as fuel, various more beneficial applications are possible. The usage of Organosolv lignin as a polyphenol source is just one economically interesting alternative premising a pure isolation.

A very promising application is the use of Organosolv lignin as a source for ammonoxidized lignins which can be applied as soil improvers. Progressive desertification in parts of the world renders this product class highly beneficial in the future. Lignin and other ligneous biomaterials can be converted into artificial humic substances by oxidative ammonolysis (ammonoxidation). The resulting N-modified lignin represents an organo-mineral fertilizer for rehabilitation of degraded soils. The special value of these modified lignins is the slow release of nitrogen due to different binding forms of nitrogen generated upon ammonoxidation. The fact, that substantially delignified renewable materials are in addition much easier converted to degradation products such as bioethanol and biogas, led us to the development of an organosolv-based bioconversion process maximizing the isolated amount of pure lignin. This can be achieved by applying high pressure ideally with carbon dioxide. As the subsequent ammonoxidation is usually executed in aqueous media, a significant amount of energy can be saved by avoiding the intermediate purification of the lignin. Therefore we combined this two process steps in one by performing the ammonoxidation already during the Organosolv process replacing the water with aqueous ammonia.

Presented topics at the conference

a.    Development of an Organosolv process aligned for maximal lignin output

The developed process enables to isolate major parts of the total lignin content in rye straw. Significant increase of the lignin output can be achieved by applying a high pressure during the process. Highest amounts can be obtained when carbon dioxide is used during the reaction. This effect can be explained by the super critical state of the carbon dioxide at the investigated reaction conditions making the raw material better accessible and acting as a solvent enhancer. A statistical approach was used to find the optimal regions for all operating parameters as temperature, pressure, time and ammonia content. Additional tests were done to evaluate the contributions of various supplements namely acids and bases and the possible exchange of ethanol with methanol which can save up to twenty percent in the overall solvents costs. The consumption of carbon dioxide in the process has a collateral positive impact towards the eco-friendly character of this process as it can be rendered as carbon dioxide neutral. The purity of the obtained lignin fractions and the degree of delignification of the remaining starting material was analyzed by curie-point pyrolysis GC/ MS. Elementary analysis was done to quantify the amount of incorporated nitrogen. Incineration analysis of the obtained lignin was done to quantify the ash content which can be seen as an additional parameter for purity.

b.    Investigation of the reaction kinetics through model substance studies

Kinetic studies were done to understand the influence of super critical carbon dioxide on the reaction mechanism in more detail. Apocynol, a phenol derivative, was used as a simple model for lignin. Kinetics with and without super critical carbon dioxide were recorded at different temperatures between five minutes and two hours. The decay of the original model substance and the formation of degradation products were evaluated by HPLC. These studies clearly show, that the use of super critical carbon dioxide during the process accelerates not only the decay of the model substance but also lowers the activation energy after Arrhenius distinctly which is in accordance to current literature.