Production of lactic acid and methane from renewable resources : an innovative green biorefinery concept for biogas process chains

摘要

The increasing demands of world’s growing population for food, energy and products, the effects of climate change and the depletion of fossil resources forces the development of sustainable industries. Based on renewable resources, state-of-the-art processes have to be transformed to eco-friendly production sequences to lead the industry to a new, bio-based economy. An essential part of the bio-based economy will be biorefineries, as they enable the production of goods and energy from bio-based resources. The aim of this study was to establish an innovative green biorefinery concept to optimize biogas process chains. The green biorefinery concept was set up to both utilize and add value to green biomass, as well as other common raw substrates used in biogas processing, by producing platform chemicals and biogas. New ensiling techniques were applied, in order to increase the amount of valuable ingredients in the silage with a special focus on lactic acid. After solid-liquid separation of the silage to exploit organic acids, the solid residue was used for anaerobic digestion. In particular the objectives were:(1) to clarify which valuable chemicals can be increased in significant amounts, depending on the raw substrate, (2) to examine the technical, chemical and biological parameters affecting the increase of valuable products in the silage and (3) to investigate the methane formation potential of the residual biomass and the fresh silage to identify potential methane losses.Lactic acid was the most promising chemical, increased to highest amounts during the ensiling process. The addition of carbonated lime was the most effective treatment to increase the amount of lactic acid, requiring a high fermentability coefficient of the utilized raw substrate. Additional lactic acid producing bacteria can help to stabilize the silage and promote the growth of lactic acid contents. Supplying the lactic acid bacteria with additional trace elements (manganese) showed no effect on lactic acid production. The comparison of specific methane yields of the fresh silages with the corresponding solid residues always yielded higher values for the fresh silage (not always significant), due to the loss of volatile solids during the fractionating. Furthermore, there is a loss of overall methane production, due to the reduction of mass while fractionating. An initial economic assessment revealed that selected variations of the treated raw substrates maize and grass offer a huge potential for the presented biorefinery concept, as the increase in lactic acid contents was immense while simultaneously having no significant losses in specific methane yields. Crucial importance for the economic feasibility lies on the downstream process of lactic acid. Future research has to be focused on establishing adequate extraction techniques, as the extraction and purity of lactic acid is the primary challenge for the economic viability of the concept. In the context of adding value to existing biogas process chains, the presented green biorefinery concept is an alternative conversion path of biomass and will likely be of monetary interest in the near future. Moreover, the improved silages can be beneficial in other applications, such as the production of middle chain fatty acids for further processing. The presented biorefinery concept is of high value for numerous applications and shows an improved method of green biorefining, which can contribute to leading our society and industry to a sustainable and multifaceted future.