The main idea of a circular economy (CE) is to separate economic growth from resource consumption and environmental impacts. The characteristic approach of a CE assumes the minimisation of the amount of waste generated at the design level and, as a standard, includes innovations throughout the value chain. From an agricultural point of view, agricultural biogas plants are particularly important because they enable the management of all waste biomass and its conversion into useful energy and agricultural fertiliser. This paper presents methods for assessing the economic effectiveness of an investment in an agricultural biogas plant. The research goal was to develop a financial model. The authors of this study used available examples of the profitability of commercial ventures. We considered the investment aspects of agricultural biogas plants. Exemplary solutions are discussed, allowing the reader to become acquainted with various methods and proposals for thus far estimated investments. It may seem chaotic, but this is how the biogas market is characterised in the context of the implementation of biogas projects. Guidance is given regarding how to understand investing in this sensitive private farming sector. It is admirable that the renewable energy market has been systematised, and we hypothesise that it is necessary to develop an investment model in Polish conditions.

Biohydrogen production is a promising alternative to replace fossil fuels in an environmentally friendly way. In addition to the many available renewable energy sources, the production of “colored” hydrogen and biohydrogen occupies an irreplaceable position due to the undeniable availability of biomass and the need to manage food waste (FW). This article presents the current state of biohydrogen production technology—examples on continents (America, Africa, Asia, Australia and Oceania) and in Europe in terms of the efficiency of dark methane fermentation (CH₄). Biophotolysis processes leading to the production of biohydrogen are indicated: directly and indirectly. The mechanism of the fermentation process of obtaining hydrogen and two-stage hydrogen fermentation are presented. The novelty of this article is the development of innovative trends in the development of the biohydrogen industry in Europe. Various models of the biohydrogen process are presented for different raw materials and proportions of substrates used in co-fermenters. Researchers from China are the undisputed pioneers in the use of renewable energy sources. However, improved energy self-sufficiency and environmental impacts are reflected in the growing number of pilot installations operating in European countries. This also gives hope for rapid progress towards full animal and FW management also in Poland.

This paper describes examples of energy crops that are profitable to burn, and whose cultivation is not complicated or expensive. Rapid growth of biomass, especially the green mass of energy crops, is indicated, which means that, in relation to fossil fuels, energy crops are considered renewable raw materials. An assessment of Polish non-food energy agriculture was conducted in the context of the prospects of a renewable energy source, namely, biomass. Recommendations for crop cultivation, the size of possible yields and the most important parameters of the obtained biomass, which have the greatest impact on the suitability of energy use, are presented. Materials of biological origin for combustion are divided into three groups: wood waste, by-products and plant products for the energy industry. It is indicated that 2 tons (Mg) of dry wood or straw is energetically equivalent to 1 Mg of coal, and 1 m³ of biogas is energetically equivalent to 1 kg of Polish coal. A novelty of this article is the interpretation of obtaining primary energy, including energy from renewable sources, in the European Union and Poland, taking into account the production of wood waste, straw, cereals and energy crops. The mechanism of the impact of the production parameters of energy crops was revealed during the prepared analysis of the prospects of Polish energy agriculture. Additionally, we conducted an analysis of the potential of biomass as a source of energy in the context of: obtaining primary energy, including energy from renewable sources, in the European Union and Poland; the number of biogas plants in Poland; and the area of agricultural land that is potentially useful for the cultivation of energy crops.

This article concerns the method of material consumption assessment of the cylinder-piston group of diesel engines in the biodiesel environment. The obtained experimental dependences of the wear coefficients on the example of the tribounit cylinder liner and the piston ring can be used to forecast the resource use during operation under specific conditions of the engine and the environment as a whole. The article systematizes the types of biofuels, depending on the type of raw materials from which they were made, taking into account the process and application. The physical and chemical aspects of the catalysts used for biofuels were indicated. The applied experimental methods for tribological wear of the piston-cylinder pair were analyzed. B70 biodiesel was used in the research, i.e., 70% mineral diesel oil and 30% methyl esters of rapeseed oil. Experimental tribotechnical studies of the influence of biofuels on the behavior of various materials have shown that when using this type of fuel, it is necessary to replace the materials from which some parts of the cylinder-piston group are made. To solve this problem, research has been carried out on a specially designed friction machine. The novelty in the article concerns the association, based on the literature, of hydrogen consumption causing material wear in friction contacts. The mechanism of the interaction of various construction materials during such friction has been disclosed.

The article reviews selected methods and techniques of agricultural biogas desulphurization. Presented is the current state of technological and measurement systems as well as raw biogas purification methods in terms of control and measurement-socio-economic aspects were also pointed out. On the example of a pilot agricultural biogas with the use of pig slurry, the required technical and technological criteria for the production and processing of agricultural biogas were indicated. The article presents the preliminary results of experimental studies on the course of changes in the volumetric composition of biogas on the basis of the average daily production of agricultural biogas.The amount of H₂S in raw and purified biogas was analyzed with the proprietary biogas desulphurization method in terms of the process parameters. A novelty is the use of a developed carbon mixture (activated carbon) with turf ore (iron compounds), which allows for 100% desulfurization of raw agricultural biogas under process conditions for mesophilic fermentation. The measurement results show a clear influence of desulphurization using the proprietary adsorption-absorption technique-agricultural biogas.

The properties, types, and physical and chemical aspects of pig slurry used in the fermentation process were presented. Characterization of the pig slurry microflora for a controlled biogas production process was performed. A pilot biogas treatment installation was presented on the example of a farm with 1100 Dan Bred fatteners kept in a grate system. The research was carried out to measure the biogas flow rate resulting from the reference pressure in the fermentor. An independent assessment of the amount of biogas and the pressure drop in the skeletal deposit was carried out. The basis for assessing the hydrodynamics of gas flow through the adhesive bed is the flow characteristic, which results from the pressure that forces this flow. In each case, the determination of this characteristic consists in determining the influence of the biogas stream on the value of this overpressure, equivalent to the pressure drop (it is tantamount to determining the total biogas flow resistance through the adhesive bed). The results of the measurements indicate the practical application of pig slurry-a substrate in a polydisperse system for the production of agricultural biogas in the context of renewable energies. The article indicates that the ferment was periodically mixed during the day, together with the fermentation of the ferment with fresh substrate. The tests were conducted for 49 days, thus demonstrating that it is more advantageous to mix the ferment hydrodynamically, obtaining a CH₄ level of about 80%.