Lignocellulosic biorefinery with a thermo-biological pretreatment concept

There is great interest, not only in technologically advanced societies, in developing innovative sources of raw materials, especially lignocellulosic biomasses such as cereal straw, which do not interfere with food production. In Germany, 4 – 9 million tons of cereal straw, mainly wheat, are utilized annually, corresponding to an energy potential of 57 – 129 PJ/a. Lignocellulose, found in plant cell walls, consists of hemicellulose, cellulose, and lignin, and it requires complex fractionation to enable its polymers to be utilized for energy or chemical applications. The sustainable use of straw is essential to reducing dependence on fossil fuels and overcoming global challenges such as climate change. This dissertation focuses on enhancing wheat straw utilization through eco-friendly thermo- biological pretreatment processes with natural additives, improving biodegradability and combustion properties. It emphasizes the role of wheat straw in biogas and solid fuel production, and includes a techno-ecological evaluation of the developed processes in a laboratory scale. The high lignocellulose content of wheat straw challenges biodegradability. Various thermo- biological pretreatment methods with additives like liquid digestate and green waste compost are studied. Compost enhances biodegradability, boosting methane production (270 – 320 L kg-1 oTS-1) by reducing technical digestion time and metabolic losses during anaerobic biological pretreatment. The unfavorable combustion properties of wheat straw known from the literature, such as low calorific value, high ash content, and low ash melting temperature, are addressed by exploring compost as an additive for straw pellet ash melting improvement, consistent with the fuel specifications of ISO 17225-6. A Life-Cycle-Assessment following ISO 14040/14044 standards compares environmental impacts of reference and developed thermo-biological processes, favoring eco-friendliness in Liquid-Hot-Water and technical advantages in acetone-based methods. The developed pretreatment processes are more advantageous than the reference processes in terms of environmental criteria, and additionally environmental savings are demonstrated in combination with thermal pressure hydrolysis. In conclusion, the cascaded use of straw–compost mixtures presents opportunities to increase the value-added potential of the biorefinery concept and optimize the utilization of feedstocks by producing gaseous and solid energy sources efficiently. This research suggests that a process- and cascade-optimized thermo-biological pretreatment process could transform landfills with connected composting and biogas plants into biorefineries, enabling the integrated material and energetic use of renewable resources and supporting the economic feasibility of a lignocellulosic biorefinery.