• Energy Research
• 7. Clean energy close

Biomass (especially algae) is a renewable energy source that can be a great alternative to fossil fuels. Wet algal biomass converts into products such as solid, aqueous, and gaseous phases as well as biocrude in hydrothermal liquefaction (HTL). The aim of this work was to provide detailed exergy analyses of the production of biocrude from Nannochloropsis sp. by HTL. Physical and chemical exergy of the HTL products, exergy losses, exergy efficiency, and exergy distribution of the HTL process were determined in this research. The highest exergy loss and the lowest efficiency values obtained for the heat exchanger were 65,856.83 MJ/hr and 66.64%, respectively, which was mainly caused by the irreversibility of the heat transfer process. Moreover, the HTL reactor had high efficiency (99.9%) due to the complex reactions that occurred at high temperature and pressure. Also, the optimum operating conditions of the reactor were obtained at 350 °C and 20 MPa by using sensitivity analysis. The high overall exergy efficiency of the process (94.93%) indicated that HTL was the most effective process for the conversion of algae. In addition, the exergy recovery values of the overall exergy input values in the HTL process for biocrude, as well as the aqueous, solid, and gas phases, were nearly 74.88%, 18.42%, 0.86%, and 0.76%, respectively. Exergy assessment provides beneficial information for improving the thermodynamic performance of the HTL system.

Abstract The paper quantifies the synergy-effects of an areal combination of biogas-plants with plants of the building materials industry (e.g. cement works) from the energetic and economical point of view. Therefore an overall process model based on energy and mass flow balances is developed to determine the effects of a combination of both plants in terms of energetic efficiency, investment and operating costs, greenhouse gas emission reduction and overall energy production costs. The results and the calculation procedure for a combination of biogas plants with cement works are presented in detail. The main benefits of this combination are the utilisation of low temperature excess heat sources from cement works for fermenter heating and the direct thermal utilisation of unprocessed biogas as a valuable, CO2-neutral fuel for combustion processes for instance clinker burning. Due to the combination, the energetic efficiency of the biogas plant, defined as utilisable energy output in relation to the energy content of the produced biogas, significantly increases from 63.0% to 83.8%. Concurrently the energy production costs are reduced, turning biogas into a competitive source of energy without the need for federal sponsorship. Calculations show, that production costs in combined plants for plant sizes larger than 90 m³STP/h biogas are even lower than the actual market prize of natural gas.

Abstract Hydrothermal liquefaction (HTL) is a promising technology for the production of renewable biocrude from a variety of biomass. Experiments with different biogenic wastes such as sewage sludge, food and green waste and grease residue were performed to assess their suitability for hydrothermal liquefaction either as individual material or in physical mixtures with microalgae biomass (= hydrothermal co-liquefaction). The experiments were carried out in a high pressure batch autoclave at temperatures of 350 °C, a holding time of 15 min in nitrogen-atmosphere and a dry matter content of 10%. Mass balances revealed the yields of the four HTL-phases (biocrude, gaseous, solid and water phase). Additional analyses comprise Soxhlet-extraction to determine the lipid content of the raw materials, FTIR spectroscopy for HTL-gas phase composition as well as determination of calorific values and elemental compositions for raw material and respective biocrude samples. The following biocrude yields were achieved for the individual raw materials: green waste - 4.4%; sewage sludge - 12.0%; food waste - 18.2%; grease residue - 76.3%; chlorella vulgaris - 18.3%. Co-liquefaction effects for binary physical mixtures (50:50) of the biogenic residues with chlorella vulgaris could not be observed on a statistically significant basis. Hence, for the investigated samples, co-liquefaction is an option to reduce logistics costs and ensure sufficient and constant raw material mixtures for processing rather than increasing the biocrude yields by synergistic effects during Co-HTL.

Abstract Background Due to climate change and the rising world population, sustainable energy and fertilizer production faces many challenges. The utilization of organic waste fractions is one possible solution for promoting sustainability. Organic waste fractions have a high potential for biomethane production, which could positively contribute to the current energy mix. Furthermore, organic waste fractions could be used for nutrient recovery (i.e., the recovery of N and P) concurrently to their use in biomethane production. This study examined the theoretical potential of organic waste fractions for valorization in Austria. Further, it provides a theoretical overview of biomethane production and nutrient-recovery potential. Results This analysis revealed a total substrate potential of 13 Mt per year in Austria, with the highest contribution from manure. Over 900 million Nm3 of biomethane could potentially be produced from organic waste fractions. Furthermore, developing organic waste fractions as an energy source could improve the impact of the natural gas consuming sectors on climate, reducing 2.4 Mt of CO2 emissions annually. Regarding nutrient recovery, more than 60 kt of N and 20 kt of P could potentially be recovered per year. Conclusion The study shows a high potential for producing biomethane from organic waste fractions in Austria. The overall production potential could substitute up to 11% of the Austrian natural gas demand, which could highly decrease the CO2 emissions from fossil energy carriers. Furthermore, a high nutrient recovery potential was identified for an inclusive implementation of an efficient recovery.