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Biogas is an instrument of synergy between responsible waste management and renewable energy production in the overall transition to sustainability. The aim of this research is to assess the integration of the power-to-gas concept into a food waste-based biogas plant with the goal to produce renewable methane. A robust optimisation was studied, using linear programming with the objective of minimising total costs, while considering the market price of electricity. The mathematical model was tested at an existing biogas power plant with the installed capacity of 1 MWel. It was determined that the integration of power-to-gas in this biogas plant requires the installation of ca. 18 MWel of wind and 9 MWel of photovoltaics, while importing an additional ca. 16 GWhel from the grid to produce 36 GWh of renewable methane. The economic analysis showed that the feedstock gate fee contributes significantly to the economic viability of renewable methane: a change in the feedstock gate fee by 100 €/tonne results in a decrease of production costs by ca. 20–60%. The robust nature of the model showed that uncertainties related to electricity production from wind and photovoltaics at the location increased the cost of gas production by ca. 10–30%.

The aim of this study is to investigate the operation of biogas plants in advanced energy markets after energy crops become limited in their use and biogas plants exit subsidy schemes for electricity production. Continuous biogas combined heat and power production and sale of electricity on the day-ahead market could be a viable operation strategy only in the case of low-cost substrates. When the break-even cost of electricity production in biogas power plants reaches 100 €/MWhel, selling electricity on the day-ahead market does not create profit. The study shown that a more profitable operation strategy involves coupling biogas power plant operation on the electricity balancing market with biomethane production or combining a small-scale sugar beet processing facility with a biogas upgrading plant to cover heat demand for sugar beet processing. Techno-economic analysis showed that the viability of both alternative operation strategies is severely impacted by the selling price of biomethane. In the given market conditions, a selling price of biomethane below 50 €/MWh is not viable for a biogas plant. The model developed could be used as a guideline for biogas plant operators on how to proceed after significant changes appear in both biogas production and biogas utilisation.

Large amounts of food waste and sewage sludge exert a hazardous environmental impact in several countries. Producing biogas and digestate from food and industrial waste is one of the solutions for waste management, stabilization of sludge, resource and energy recovery and reductions in the amount of waste. However, biogas production from such substrates has challenges in degradation efficiency, inhibitory effects and other challenges, and thus co-digestion and pretreatment techniques could be applied to enhance biogas production. The aim of this study is to explore the effects of co-digestion of food waste, meat and bone meal and rendering wastewater sludge. First, thermal pretreatment was performed (35°C, 5 days) by adding the rendering-industry streams to food waste in the amounts of 0, 5, 10 and 15% on a total solid basis, and further anaerobic digestion (40.5°C, ca. 40 days) was then performed. Both experimental and kinetic analysis were conducted, and the major factors regarding opportunities and challenges in the two-stage process are discussed. Results have shown that both co-substrates from rendering industry decreased the biogas yield of food waste. When 5% of them was added to food waste, meat and bone meal decreased biogas production by 12%, and wastewater sludge decreased it by 23%. Both co-substrates, on the other side, increased the rate of reaction of food waste digestion when applying different common kinetic models.

Sustainable management in the biogas production via anaerobic digestion process intents the use of alternative biomass sources that are not competitive with food production. The aim of this study is to investigate the application of the abundant-quantity residue in more sustainable production of heat and electricity along with the production of the digested substrate as a fertiliser. The study has been divided into several sequential steps. First, the grass samples have been collected at the following locations: uncultivated land, river embankment and highway verge. The greatest grass yield has been determined for the riverbank grass, with an average value of 19 t/ha of fresh mass and 2.6 t/ha of dry mass. Next, the chemical characterisation of the collected residue grass and the laboratory batch mono and co-digestion tests with maize silage and cattle slurry have been conducted. The results show that all grass samples have satisfying digestive parameters (C/N ratio between 16.6:1 to 22.8:1) with the low presence of impurities, which makes them suitable for biogas production. The following biochemical methane potential in mono-digestion of residue grass has been recorded: uncultivated land (0.275 Nm3/kgTS), riverbank (0.192 Nm3/kgTS) and highway verge (0.255 Nm3/kgTS). The control of the process has been improved in co-digestion tests, by avoiding acidification in the first days of the operation. The estimation of kinetic parameters in mathematical modelling has shown that the degradation of residue grass shows some different parameters compared to the previous study. The model results for the gas phase show some small deviations compared to the experimental data. Based on the life cycle analysis results it can be concluded that there are perspectives in the use of residual grass compared to maize silage in the production of heat and electricity, especially in the improvement of ecosystem quality.

The aim of the paper is to examine the possibility of application of the spreadsheet calculator and Reference Fluid Thermodynamic and Transport Properties database to a thermodynamic process. The heating process of a real soil-to-water heat pump, including heat transfer in the borehole heat exchanger has been analysed. How the changes of condensing temperature, at constant evaporating temperature, influence the following: heating capacity, compressor effective power, heat supplied to evaporator, compression discharge temperature and coefficient of performance, are investigated. Also, the energy characteristics of a heat pump using different refrigerants for the same heating capacity and the same temperature regime are compared. The following refrigerants are considered: two zeotropic mixtures, R407C and R409A, a mixture with some zeotropic characteristics, R410A, and an azeotropic mixture, R507A.